The Ancestor's Tale Part 17

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Placozoans join. As with Rendezvous 28 Rendezvous 28 and and 29 29, the order of Rendezvous 30 Rendezvous 30 and and 31 31 is pretty well unresolved. is pretty well unresolved. Rendezvous 30 Rendezvous 30 could be with the placozoans (rep-resented by its single species, could be with the placozoans (rep-resented by its single species, Trichoplax Trichoplax), or it could be with the sponges. At present this ordering is essentially arbitrary. It would be entirely unsurprising if Rendezvous 30 Rendezvous 30 and and 31 31 had to swap. had to swap.

Image: Trichoplax adhaerens Trichoplax adhaerens.

With such a weight of multi-volumed authority bearing down upon it, what chance had poor little Trichoplax Trichoplax, especially given that n.o.body had looked at the animal itself for more than half a century? It languished as an alleged cnidarian larva until the molecular revolution opened up the possibility of discovering its real affinities. Whatever else it is, it is definitely not a cnidarian. Preliminary indications from rRNA studies (see Taq's Tale) suggest that Trichoplax Trichoplax is more distant from the rest of the animal kingdom than any other group except the sponges, and it may be that even the sponges are closer to us than is more distant from the rest of the animal kingdom than any other group except the sponges, and it may be that even the sponges are closer to us than Trichoplax Trichoplax is. Trichoplax has the smallest genome and the simplest bodily organisation of any multicellular animal. It has only four cell types in its body, compared to more than 200 in us. And it appears to have a single Hox gene. is. Trichoplax has the smallest genome and the simplest bodily organisation of any multicellular animal. It has only four cell types in its body, compared to more than 200 in us. And it appears to have a single Hox gene.

Molecular genetic evidence indicates provisionally that this lonely little pilgrim joins us at Rendezvous 30 Rendezvous 30, perhaps 780 million years ago, 'before' the sponges. But this is really anyone's guess. It could be that Rendezvous 30 Rendezvous 30 and and 31 31 (sponges) should be reversed, in which case (sponges) should be reversed, in which case Trichoplax Trichoplax is our most distant cousin among the true animals. Understandably, there is now some strong lobbying for is our most distant cousin among the true animals. Understandably, there is now some strong lobbying for Trichoplax Trichoplax to join that select company of organisms whose genome is completely sequenced. I think it will happen, in which case we should soon know what this strange little creature really is. to join that select company of organisms whose genome is completely sequenced. I think it will happen, in which case we should soon know what this strange little creature really is.

Rendezvous 31 SPONGES.

Sponges are the last pilgrims to join us who are members of the Metazoa, the truly multicellular animals. Sponges haven't always been dignified as Metazoa, but were written off as 'Parazoa' a name for a kind of second-cla.s.s citizen of the animal kingdom. Nowadays the same cla.s.s distinction is fostered by placing the sponges in the Metazoa, but coining the word Eumetazoa for all the rest except except sponges (some authors also except sponges (some authors also except Trichoplax Trichoplax, the little animal we met at Rendezvous 30 Rendezvous 30).

People are occasionally surprised to learn that sponges are animals rather than plants. Like plants, they don't move. Well, they don't move their whole body. Neither plants nor sponges have muscles. There is movement at the cellular level, but that is true of plants too. Sponges live by pa.s.sing a ceaseless current of water right through the body, from which they filter food particles. Consequently, they are full of holes, which is what makes them so good at holding water in the bath.

Bath sponges, however, don't give a good idea of the typical body form, which is a hollow pitcher with a big opening at the top and lots of smaller holes all round the sides. As is easy to tell by putting a little dye in the water outside the pitcher of a living sponge, water is drawn in through the small holes around the sides, and expelled into the main hollow interior, from which it flows out through the main entrance of the pitcher. The water is driven by special cells called choanocytes, which line the chambers and ca.n.a.ls of the walls of a sponge. Each choanocyte has a waving flagellum (like a cilium, only larger) surrounded by a deep collar. We shall meet the choanocytes again, as they are important for our evolutionary story.

Sponges have no nervous system and a relatively simple internal structure. Although they have several different kinds of cells, those cells don't organise themselves into tissues and organs the way ours do. Sponge cells are 'toti potent', which means that every cell is capable of becoming any of the sponge's repertoire of cell types. This is not true of our cells. A liver cell is not capable of giving rise to a kidney cell or a nerve cell. But sponge cells are so flexible that any isolated cell is capable of growing a whole new sponge (and there's more to it, as we shall see in the Sponge's Tale).

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Sponges join. Since the time of Linnaeus, the animals ('metazoans') have been cla.s.sified as one of the kingdoms of life. The approximately 10,000 described species of sponge are usually seen as a very early diverging branch, a position confirmed by molecular data (though Since the time of Linnaeus, the animals ('metazoans') have been cla.s.sified as one of the kingdoms of life. The approximately 10,000 described species of sponge are usually seen as a very early diverging branch, a position confirmed by molecular data (though Trichoplax Trichoplax may have diverged even earlier). A minority of molecular taxonomists think that there are two lineages of sponges, one more closely related to the rest of the Metazoa than the other this implies that the earliest metazoans really did look like sponges and would have been cla.s.sified as such but this is highly controversial. may have diverged even earlier). A minority of molecular taxonomists think that there are two lineages of sponges, one more closely related to the rest of the Metazoa than the other this implies that the earliest metazoans really did look like sponges and would have been cla.s.sified as such but this is highly controversial.

Image: yellow tube sponge ( yellow tube sponge (Aplysina fistularis).

Unsurprisingly, therefore, sponges make no distinction between 'germ line' and 'soma'. In the Eumetazoa, germ-line cells are those that are capable of giving rise to reproductive cells and whose genes are therefore in principle immortal. The germ line is a small minority of cells residing in ovaries or testes, and insulated from the need to do anything else but reproduce. Soma is that part of the body that is not germ line somatic cells are destined not to pa.s.s their genes on indefinitely. In a eumetazoan such as a mammal, a subset of cells is set aside early in embryology as germ line. The rest of the cells, the cells of the soma, may divide a few times to make liver or kidney, bone or muscle, but then their dividing career comes to an end.

Cancer cells are the sinister exception. They have somehow lost the ability to stop dividing. But as Randolph Nesse and George C. Williams, the authors of The Science of Darwinian Medicine The Science of Darwinian Medicine, point out, we should not be surprised. On the contrary, the surprising thing about cancer is that it is not more common than it is. Every cell in the body, after all, is descended from an unbroken line of billions of generations of germ-line cells that have not stopped dividing. Suddenly being asked to become a somatic cell like a liver cell, and learn the art of not not dividing, has never happened before in the entire history of the cell's ancestors! Don't be confused. Of course the bodies that housed the cell's ancestors had livers. But germ-line cells by definition are not descended from liver cells. dividing, has never happened before in the entire history of the cell's ancestors! Don't be confused. Of course the bodies that housed the cell's ancestors had livers. But germ-line cells by definition are not descended from liver cells.

All sponge cells are germ-line cells all potentially immortal. They have several different cell types, but they are deployed in development in a different way from most multicellular animals. Eumetazoan embryos form cell layers that fold and inv.a.g.i.n.ate in complicated 'origami' ways to build the body. Sponges don't have that kind of embryology. Instead they self-a.s.semble each of their toti-potent cells has an affinity for hooking up to other cells, as though they were autonomous protozoa with sociable tendencies. Nevertheless, modern zoologists include the sponges as members of the Metazoa, and I am following this trend. They are probably the most primitive living group of multicellular animals, giving us a better idea of the early Metazoa than any other modern animals.

As with other animals, each species of sponge has its own characteristic shape and colour. The hollow pitcher is only one of many forms. Others are variants on it, systems of hollow cavities connected to one another. Sponges characteristically toughen their structures with collagen fibres (that's what makes bath sponges spongy) and with mineral spicules: crystals of silica or calcium carbonate, the shape of which is often the most reliable diagnostic of the species. Sometimes the spicule skeleton can be intricate and beautiful, as in the gla.s.s sponge, Euplectella Euplectella (see plate 44) (see plate 44).

The date of Rendezvous 31 Rendezvous 31 is given as 800 million years on the phylogeny diagram, but the usual despairing warnings for such ancient datings apply. The evolution of multicellular sponges from single-celled protozoa is one of the landmark events in evolution the Origin of the Metazoa and we shall examine it in the next two tales. is given as 800 million years on the phylogeny diagram, but the usual despairing warnings for such ancient datings apply. The evolution of multicellular sponges from single-celled protozoa is one of the landmark events in evolution the Origin of the Metazoa and we shall examine it in the next two tales.

THE SPONGE'S TALE The 1907 issue of the Journal of Experimental Zoology Journal of Experimental Zoology contains a paper on sponges by H. V. Wilson of the University of North Carolina. The research was cla.s.sic, and the paper describing it recalls a golden age when scientific papers were written in a discursive style that you could understand, and at a length that made it possible to visualise a real person doing real experiments in a real laboratory. contains a paper on sponges by H. V. Wilson of the University of North Carolina. The research was cla.s.sic, and the paper describing it recalls a golden age when scientific papers were written in a discursive style that you could understand, and at a length that made it possible to visualise a real person doing real experiments in a real laboratory.

Wilson took a living sponge and separated its cells by forcing them through a fine sieve a piece of 'bolting cloth'. The disa.s.sembled cells were pa.s.sed into a saucer of sea water, where they formed a red cloud, mostly consisting of single cells. The cloud settled down into a sediment at the bottom of the saucer, where Wilson observed them with his microscope. The cells behaved like individual amoebas, crawling over the bottom of the saucer. When these amoeboid crawlers met others of their kind, they joined up to form growing agglomerations of cells. Eventually, as Wilson and others showed in a series of papers, such agglomerations grow to become whole new sponges. Wilson also tried mas.h.i.+ng up sponges of two different species and mixing the two suspensions together. The two species were of different colours, so he could easily see what happened. The cells chose to agglomerate with their own species and not the other. Oddly, Wilson reported this result as a 'failure', since he was hoping for reasons I don't understand, and which perhaps reflect the different theoretical preconceptions of a zoologist of nearly a century ago that they would form a composite sponge of two different species.

The 'sociable' behaviour of sponge cells as exhibited by such experiments perhaps sheds light on the normal embryonic development of individual sponges. Does it also give us some sort of hint of how the first multicellular animals (metazoans) evolved evolved from single-celled ancestors (protozoans)? The metazoan body is often called a colony of cells. In keeping with this book's pattern of using some tales as modern re-enactments of evolutionary happenings, could the Sponge's Tale be telling us something about the remote evolutionary past? Could the behaviour of the crawling and agglomerating cells in Wilson's experiments represent some sort of re-enactment of how the first sponge arose as a colony of protozoans? from single-celled ancestors (protozoans)? The metazoan body is often called a colony of cells. In keeping with this book's pattern of using some tales as modern re-enactments of evolutionary happenings, could the Sponge's Tale be telling us something about the remote evolutionary past? Could the behaviour of the crawling and agglomerating cells in Wilson's experiments represent some sort of re-enactment of how the first sponge arose as a colony of protozoans?

Almost certainly it was not the same in detail. But here is a hint. The most characteristic cells of sponges are the choanocytes, which they use for generating currents of water. The picture shows a portion of the wall of a sponge, with the inside of the cavity to the right. The choanocytes are the cells that line the cavity of the sponge. 'Choano-' comes from the Greek for 'funnel', and you can see the little funnels or collars, made up of many fine hairs known as microvilli. Each choanocyte has a beating flagellum, which draws water through the sponge, while the collar catches nutrient particles in the stream. Take a good look at those choanocytes, for we shall meet something rather like them at the next rendezvous. And then, in the light of that, the following tale will complete our speculation about the origin of multicellularity.

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Sociable cells. Portion of sponge wall showing choanocytes, with their distinctive collars and flagella. Portion of sponge wall showing choanocytes, with their distinctive collars and flagella.

Rendezvous 32 CHOANOFLAGELLATES.

The choanoflagellates are the first protozoans to join our pilgrimage, and they do so at Rendezvous 32 Rendezvous 32, which, very tentatively on molecular evidence with worryingly large extrapolations, we date at 900 million years. Look at the picture below. Do the little flagellated cells remind you of anything? Yes, they are very similar to the choanocytes lining the water ca.n.a.ls of sponges. It has long been suspected that either they represent a hangover from a sponge ancestor, or they are the evolutionary descendants of sponges that have degenerated to single cells or very few cells. Molecular genetic evidence suggests the former, which is why I am considering them as separate pilgrims, joining our pilgrimage here.

There are about 140 species of choanoflagellates. Some are free-swimming, propelling themselves along with the flagellum. Others are attached by a stalk, sometimes several together in a colony, as in the picture. They use their flagellum to drive water into the funnel, where food particles such as bacteria are trapped and engulfed. In this respect they are different from the choanocytes of sponges. In a sponge, each flagellum is used not to drive food into the individual funnel of the choanocyte, but in co-operation with other choanocytes to draw a current of water in through holes in the walls of the sponge and out through the sponge's main opening. But anatomically each individual choanoflagellate, whether it is in a colony or not, is suspiciously similar to a sponge choanocyte. This fact will bulk large in the Choanoflagellate's Tale, which resumes the topic begun by the Sponge's Tale: the origin of multicellular sponges.

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Is this how it was? A colony of choanoflagellates. A colony of choanoflagellates.

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Choanoflagellates join. The 120 or so known species of choanoflagellate are conventionally seen as close relatives of the animals, a position strongly supported by both morphological and molecular data. The 120 or so known species of choanoflagellate are conventionally seen as close relatives of the animals, a position strongly supported by both morphological and molecular data.

Image: Codosiga gracilis Codosiga gracilis.

THE CHOANOFLAGELLATE'S TALE Zoologists have long enjoyed speculating about how multi-cellularity evolved from protozoan ancestors. The great nineteenth-century German zoologist Ernst Haeckel was one of the first to propose a theory of the origin of the Metazoa, and some version of his theory is still much favoured today: the first metazoan was a colony of flagellate protozoa.

We met Haeckel in the Hippo's Tale, in connection with his prescient linking of hippos with whales. He was a pa.s.sionate Darwinian, who made a pilgrimage to Darwin's house (which the great man found irksome). He was also a brilliant artist, a dedicated atheist (he sardonically called G.o.d a 'gaseous vertebrate'), and a particular enthusiast for the now unfas.h.i.+onable theory of recapitulation: 'Ontogeny recapitulates phylogeny', or 'The developing embryo climbs up its own family tree.'

You can see the appeal of the idea of recapitulation. The life story of every young animal is a telescoped re-enactment of its (adult) ancestry. We all start as a single cell: that represents a protozoan. The next stage in development is a hollow ball of cells, the blastula. Haeckel suggested that this represents an ancestral stage, which he called the blastaea. Next in embryology, the blastula inv.a.g.i.n.ates, like a ball punched in as a dent from one side, to form a cup lined by a double layer of cells, the gastrula. Haeckel imagined a gastrula-stage ancestor, which he called the gastraea. A cnidarian, such as a hydra or a sea anemone, has two layers of cells, like Haeckel's gastraea. In Haeckel's recapitulationist view, cnidarians stop climbing up their family tree when they reach the gastrula stage, but we soldier on. Subsequent stages in our embryology resemble a fish with gill slits and a tail. Later we lose our tail. And so on. Each embryo stops climbing up its family tree when it reaches its appropriate evolutionary stage.

Appealing as it is, the recapitulation theory has become unfas.h.i.+onable or rather it is now regarded as a small part of what is sometimes but not always true. The whole matter is thoroughly discussed in Stephen Gould's book Ontogeny and Phylogeny Ontogeny and Phylogeny. We must leave it there, but it is important for us to see where Haeckel was coming from. From the point of view of the origin of the Metazoa, the interesting stage in Haeckel's theory is the blastaea: the hollow ball of cells that, in his view, was the ancestral stage now reprised in embryology as the blastula. What modern creature can we find that resembles a blastula? Where shall we find an adult creature that is a hollow ball of cells?

Setting aside the fact that they are green and photosynthesise, the group of colonial algae called the Volvocales seemed almost too good to be true. The eponymous member of the group is the largest, Volvox Volvox itself, and Haeckel could hardly have wished for a neater model blastaea than itself, and Haeckel could hardly have wished for a neater model blastaea than Volvox Volvox. It is a perfect sphere, hollow like a blastula, with a single layer of cells, each resembling a unicellular flagellate (which happens to be green).

Haeckel's theory did not have the field to itself. In the mid-twentieth century a Hungarian zoologist called Jovan Hadzi proposed that the first metazoan was not round at all, but elongated like a flatworm. His contemporary model for the first metazoan was an acoelomorph worm of the kind we met at Rendezvous 27 Rendezvous 27. He derived it from a ciliate protozoan (we shall meet them at Rendezvous 37 Rendezvous 37) with many nuclei (which some of them have to this day). It crawled along the bottom with its cilia, as some small flatworms do today. Cell walls appeared between the nuclei, turning an elongate protozoan with one cell but many nuclei (a 'syncitium'), into a creeping worm with many cells, each with its own nucleus the first metazoan. On Hadzi's view the round metazoans such as cnidarians and ctenoph.o.r.es secondarily lost their elongated worm shape and became radially symmetrical, while most of the animal kingdom continued to expand upon the bilateral worm shape in ways that we see all around us.

Hadzi's ordering of the rendezvous points would, therefore, be very different from ours. The rendezvous with the cnidarians and ctenoph.o.r.es would come earlier in the pilgrimage than the rendezvous with the acoelomorph flatworms. Unfortunately, modern molecular evidence goes against Hadzi's ordering. Most zoologists today support some version of the Haeckel 'colonial flagellate' theory against the Hadzi 'syncitial ciliate' theory. But attention has switched away from the Volvocales, elegant as they are, and to the group whose tale this is, the choanoflagellates.

One type of colonial choanoflagellate is so sponge-like it is even called Proterospongia Proterospongia. The individual choanoflagellates (or should we stick our necks out and call them choanocytes?) are embedded in a matrix of jelly. The colony is not a ball, which would not have pleased Haeckel, although he appreciated the beauty of the choanoflagellates, as his wonderful drawings of them show. Proterospongia Proterospongia is a colony of cells of a type almost indistinguishable from those that dominate the interior of a sponge. The choanoflagellates marginally get my vote as the most plausible candidates for a recent re-enactment of the origin of the sponges, and ultimately of the whole group of Metazoa. is a colony of cells of a type almost indistinguishable from those that dominate the interior of a sponge. The choanoflagellates marginally get my vote as the most plausible candidates for a recent re-enactment of the origin of the sponges, and ultimately of the whole group of Metazoa.

The choanoflagellates would once have been lumped with all the remaining organisms who have not yet joined our pilgrimage, as 'Protozoa'. Protozoa doesn't work any more as the name for a phylum. There are lots of different ways of being a single-celled organism (or, as some would prefer, acellular having a body not divided into const.i.tuent cells). Different members of the group formerly known as Protozoa will now be joining our pilgrimage in DRIPs and drabs, separated by major contingents of multicellular creatures such as fungi and plants. I shall continue to use the word protozoan as an informal name for a single-celled eukaryote.

Rendezvous 33 DRIPS.

There is a small group of single-celled parasites known as either Mesomycetozoea or Ichthyosporea, mostly parasites of fish and other freshwater animals. The name Mesomycetozoea suggests an a.s.sociation with both fungi and animals, and it is true that their rendezvous with us animals is our last before we all join the fungi. This fact is now known from molecular genetic studies, which unite what had hitherto been a rather miscellaneous set of single-celled parasites, both with each other and with animals and fungi.1 Both 'Mesomycetozoea' and 'Ichthyosporea' are quite hard to remember, and there is disagreement over which of them to prefer. This may be why a practice has grown up of using the nickname DRIPs an acronym from the initial letters of the only four genera known to the discoverers of the group. The genera that provide the D, the I and the P are Dermocystidium, Ichthyophonus Dermocystidium, Ichthyophonus and and Psorospermium Psorospermium. The R was always a bit of a cheat because it is not a Latin name. It stood for 'Rosette agent', a commercially important parasite of salmon, now formally named Sphaerothec.u.m destruens Sphaerothec.u.m destruens. So I suppose the acronym should really have been amended to DIPS, or DIPSs in the plural. But DRIPs with an s for plural seems to have stuck. And now, with what seems like the workings of nomenclatural providence, another organism, whose name happens to begin with R, has recently been discovered to be a DRIP too. This is Rhinosporidium seeberi Rhinosporidium seeberi, a parasite of human noses. So we can redesign the name DRIPS, with all five letters being comfortably acronymical, and try to ignore the embarra.s.sing question of whether it is singular or plural.

Rhinosporidium seeberi was first discovered in 1890, and it has long been known as the cause of rhinosporidiosis, an unpleasant disease of the human, indeed mammalian, nose, but its affinities were a mystery. At different times it has been moved from protozoan pillar to fungal post, but molecular studies now show it to be the fifth DRIP. Fortunately for pun-haters, was first discovered in 1890, and it has long been known as the cause of rhinosporidiosis, an unpleasant disease of the human, indeed mammalian, nose, but its affinities were a mystery. At different times it has been moved from protozoan pillar to fungal post, but molecular studies now show it to be the fifth DRIP. Fortunately for pun-haters, R. seeberi R. seeberi doesn't seem to cause the nose to drip. On the contrary, it blocks the nostril with polyp-like growths. Rhinosporidiosis is mainly a disease of the tropics, and doctors have long suspected that people catch it by bathing in freshwater rivers or lakes. Since all other known DRIPs are parasites of freshwater fish, crayfish or amphibians, it seems likely that freshwater animals const.i.tute the primary host of doesn't seem to cause the nose to drip. On the contrary, it blocks the nostril with polyp-like growths. Rhinosporidiosis is mainly a disease of the tropics, and doctors have long suspected that people catch it by bathing in freshwater rivers or lakes. Since all other known DRIPs are parasites of freshwater fish, crayfish or amphibians, it seems likely that freshwater animals const.i.tute the primary host of R. seeberi R. seeberi too. The discovery that it is a DRIP might be helpful to doctors in other ways. For example, attempts to treat it with antifungal agents have failed, and we can now get an inkling as to why: it is not a fungus. too. The discovery that it is a DRIP might be helpful to doctors in other ways. For example, attempts to treat it with antifungal agents have failed, and we can now get an inkling as to why: it is not a fungus.

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DRIPs join. The closest single-celled relatives of the animals are the choanoflagellates and the DRIPs. It is currently uncertain whether these two groups are each other's closest relatives (so collapsing The closest single-celled relatives of the animals are the choanoflagellates and the DRIPs. It is currently uncertain whether these two groups are each other's closest relatives (so collapsing Rendezvous 32 Rendezvous 32 and and 33 33 into one), or whether the 30 or so described DRIPs species are the most distantly related to all the others. The most extensive molecular study to date supports the latter scheme, which we therefore follow. into one), or whether the 30 or so described DRIPs species are the most distantly related to all the others. The most extensive molecular study to date supports the latter scheme, which we therefore follow.

Image: Ichthyophonus hoferi Ichthyophonus hoferi.

Dermocystidium appears as cysts in the skin or gills of carp, salmonids, eels, frogs and newts. appears as cysts in the skin or gills of carp, salmonids, eels, frogs and newts. Ichthyophonus Ichthyophonus causes economically important systemic infections of more than 80 species of fish. causes economically important systemic infections of more than 80 species of fish. Psorospermium Psorospermium, which, incidentally, was originally discovered by our old friend Ernst Haeckel, infects crayfish (not fish at all, of course, but crustaceans), and again has economically important effects on crayfish stocks. And Sphaerothec.u.m Sphaerothec.u.m, as we have seen, infects salmon.

DRIPs organisms themselves would be dismissed as unremarkable, but for their evolutionarily aristocratic status their branch point, after all, is the deepest in the animal kingdom, their rendezvous with us the oldest. We don't know what Concestor 33 looked like, except insofar as single-celled organisms all look pretty much alike to our jaded multicellular eyes. It was not a parasite like a DRIP not of fish, amphibians, crustaceans or humans, that's for sure, for they all still lay unimaginably far in the future.

The one adjective that is always applied to DRIPs is 'enigmatic', and who am I to break with tradition? If a DRIP were to tell its enigmatic tale, I suspect that it would be a tale of how, now that we have reached such ancient rendezvous points, it is nearly arbitrary which of our single-celled cousins happen to have survived. Not accidentally, it is also pretty arbitrary which single-celled organisms scientists have chosen to examine at the level of molecular genetics. People have looked at DRIPs because some of them are commercially important parasites of fish, and others, we now know, bung up our noses. There could be single-celled organisms that are just as pivotal on the family tree of life, but which n.o.body has bothered to look at because they parasitise, say, komodo dragons rather than salmon or people.

n.o.body, however, could overlook the fungi. We are about to greet them.

1 Confusingly (that's putting it mildly), the name Mesomycetozoa, as opposed to Mesomycetozoea (can you spot the difference?), has been used for a more inclusive group. This seems positively designed to confuse, like the Hominoidea, Hominidae, Homininae, Homimini complex of names for our own relatives, and I prefer to boycott them all. Confusingly (that's putting it mildly), the name Mesomycetozoa, as opposed to Mesomycetozoea (can you spot the difference?), has been used for a more inclusive group. This seems positively designed to confuse, like the Hominoidea, Hominidae, Homininae, Homimini complex of names for our own relatives, and I prefer to boycott them all.

Rendezvous 34 FUNGI.

At Rendezvous 34 Rendezvous 34 we animals are joined by the second of the three great multicellular kingdoms, the fungi. The third consists of the plants. It might at first be surprising that fungi, which seem so plantlike, are more closely related to animals than they are to plants, but molecular comparison leaves little doubt. And perhaps that is not too surprising. Plants import energy from the sun into the biosphere. Animals and fungi, in their different ways, are parasites on the plant world. we animals are joined by the second of the three great multicellular kingdoms, the fungi. The third consists of the plants. It might at first be surprising that fungi, which seem so plantlike, are more closely related to animals than they are to plants, but molecular comparison leaves little doubt. And perhaps that is not too surprising. Plants import energy from the sun into the biosphere. Animals and fungi, in their different ways, are parasites on the plant world.

The fungi are a very large and important influx of pilgrims, with 69,000 species so far described out of an estimated total of 1.5 million. Mushrooms and toadstools give the wrong impression these conspicuous plant-like structures are the spore-producing tips of the iceberg. Most of the business part of the organism that made the mushroom is under the ground: a spreading network of threads called hyphae. The collection of hyphae belonging to one individual fungus is called the mycelium. The total length of mycelium of an individual fungus may be measured in kilometres, and may spread through a substantial area of soil.

A single mushroom is like a flower growing on a tree. But the 'tree', instead of being a tall, vertical structure, is spread out like the strings of a giant tennis racket underground, in the surface layers of the soil. Fairy rings are a vivid reminder of this. The circ.u.mference of the ring represents the extent of growth of a mycelium, spreading outwards from a central starting point, perhaps originally a single spore. The circular leading edge feeding edge of the expanding mycelium, the frame of the racket, is where the broken-down products of digestion are richest. These are a source of nutriment for the gra.s.s, which consequently grows more luxuriantly around the ring. Where there are fruiting bodies (mushrooms, or any of dozens of species of related fungi) they tend to grow up in the ring too.

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Fungi join. Molecular taxonomy reveals fungi to be closer to animals than to plants. The two largest groups, the Ascomycota (about 40,000 described species) and Basidiomycota (about 22,000), are usually considered closest relatives, with recent studies finding the 160 arbuscular mycorrhizal fungi to be their sister group. The groupings and branching order of the rest of the 3,000 or so fungi are not well established, particularly the number of separate branches previously lumped together in the 'Zygomycota', and the position of the microsporidians. Molecular taxonomy reveals fungi to be closer to animals than to plants. The two largest groups, the Ascomycota (about 40,000 described species) and Basidiomycota (about 22,000), are usually considered closest relatives, with recent studies finding the 160 arbuscular mycorrhizal fungi to be their sister group. The groupings and branching order of the rest of the 3,000 or so fungi are not well established, particularly the number of separate branches previously lumped together in the 'Zygomycota', and the position of the microsporidians.

Images, left to right: common morel ( common morel (Morch.e.l.la esculenta); stinkhorn (Phallus impudicus); arbuscular mycorrhizal fungus (Glomus sp.) from root of bluebell ( sp.) from root of bluebell (Hyacinthoides nonscripta); pin mould (Mucor sp.); sp.); Rhizoclostamium Rhizoclostamium sp.; sp.; Enterocytozoon bieneusi Enterocytozoon bieneusi.

Hyphae may be divided into cells by cross-walls. But sometimes they are not, and the nuclei containing the DNA are dotted along the hypha in a syncitium, meaning a tissue with many nuclei not divided into separate cells (we met other syncitia in the early development of Drosophila Drosophila, and in Hadzi's theory of the origin of the Metazoa). Not all fungi have a thread-like mycelium. Some, such as yeasts, have reverted to single cells which divide and grow in a diffuse ma.s.s. What the hyphae (or yeast cells) are doing is digesting whatever it is they are burrowing through: dead leaves and other decaying matter (in the case of soil fungi), curdled milk (in the case of cheese-making fungi), grapes (in the case of wine-making yeasts), or the grapetreader's toes (if he happens to suffer from athlete's foot).

The key to efficient digestion is to expose a large area of absorptive surface to the food. We achieve that by chewing the food into small pieces and pa.s.sing the fragments through a long coiled gut whose already large area is compounded by a forest of tiny projections, or villi, covering its lining. Each villus in turn has a brush border of hair-like micro-villi, so the total absorptive area of an adult human intestine is millions of square centimetres. A fungus such as the well-named Phallus Phallus (see plate 45) (see plate 45) or the field mushroom or the field mushroom Agaricus campestris Agaricus campestris spreads its mycelium over a similar area of soil, secreting digestive enzymes and digesting the soil material where it lies. The fungus doesn't walk about devouring food and digesting it inside its body as a pig or a rat would. Instead it spreads its 'intestines', in the form of thread-like mycelia, right through the food and digests it on the spot. From time to time hyphae come together to form a single solid structure with recognisable form: a mushroom (or toadstool, or bracket). This structure manufactures spores that float high and far on the wind, spreading the genes for making new mycelium and, eventually, new mushrooms. spreads its mycelium over a similar area of soil, secreting digestive enzymes and digesting the soil material where it lies. The fungus doesn't walk about devouring food and digesting it inside its body as a pig or a rat would. Instead it spreads its 'intestines', in the form of thread-like mycelia, right through the food and digests it on the spot. From time to time hyphae come together to form a single solid structure with recognisable form: a mushroom (or toadstool, or bracket). This structure manufactures spores that float high and far on the wind, spreading the genes for making new mycelium and, eventually, new mushrooms.

As you'd expect of a new influx of 100,000 pilgrims, they have already joined up with each other in large sub-contingents 'before' they meet us at Rendezvous 34 Rendezvous 34. All the major sub-groups of fungi end in 'mycete', the Greek for 'mushroom', which sometimes turns into 'mycota'. We have already met 'mycete' in Mesomycetozoea, the name for DRIPs that implies some sort of intermediate status between animals and fungi. The two largest and most important of these sub-contingents of the fungus pilgrims are the ascomycetes (Ascomycota) and the basidiomycetes (Basidiomycota).

The ascomycetes include some famous and important fungi, such as Penicillium Penicillium, the mould from which the first antibiotic was accidentally discovered and largely ignored by Fleming until Florey, Chain and their colleagues rediscovered it 13 years later. Incidentally, it is rather a pity that the name antibiotic has stuck. These agents attack bacteria, not viruses, and if only they had been called antibacterials instead of antibiotics, patients might stop demanding that doctors prescribe them (uselessly and even counterproductively) for viral infections. Another n.o.bel Prize-winning ascomycete is Neurospora cra.s.sa Neurospora cra.s.sa, the mould with which Beadle and Tatum developed the 'one gene one enzyme hypothesis'. Then there are the human-friendly yeasts that make bread, wine and beer, and the unfriendly Candida Candida from which we get unpleasant diseases, such as vaginitis (thrush). Edible morels and the highly prized truffles are ascomycetes. Truffles are traditionally found with the aid of female pigs, who are strongly attracted by the smell of what appears to be alpha-androstenol, a male s.e.x pheromone secreted by boars. It isn't clear why truffles produced this dead giveaway, but it may in some interesting way yet to be worked out account for their gastronomic appeal to us. from which we get unpleasant diseases, such as vaginitis (thrush). Edible morels and the highly prized truffles are ascomycetes. Truffles are traditionally found with the aid of female pigs, who are strongly attracted by the smell of what appears to be alpha-androstenol, a male s.e.x pheromone secreted by boars. It isn't clear why truffles produced this dead giveaway, but it may in some interesting way yet to be worked out account for their gastronomic appeal to us.

Most of the edible and notoriously inedible or hallucinogenic fungi are basidiomycetes: mushrooms, chanterelles, boletuses, s.h.i.+takes, ink caps, death caps, stinkhorns, bracket fungi, toadstools and puffb.a.l.l.s. Some of their spore-producing bodies can reach impressive sizes. Basidiomycetes are also of economic importance as causes of the plant diseases known as rusts and s.m.u.ts. Some basidiomycetes and ascomycetes, as well as all members of a specialised group called the glomeromycetes, collaborate with plants to supplement their root hairs with mycorrhizae, a most remarkable story, which I'll briefly relate.

We saw that the villi in our intestines and the mycelium threads of a fungus are thin, to increase the surface area for digestion and absorption. In just the same way, plants have numerous fine root hairs to increase the surface area for absorption of water and nutrients from the soil. But it is an amazing fact that most of what appear to be root hairs are no part of the plant itself. Instead, they are provided by symbiotic fungi, whose mycelium both resembles and works like true root hairs. These are the mycorrhizae, and close examination reveals that there are several independently evolved ways in which the mycorrhizal principle has been implemented. Much of plant life on our planet is utterly dependent on mycorrhizae.

In an even more impressive feat of symbiotic co-operation, basidiomycetes and independently evolved again ascomycetes form a.s.sociations with algae or cyan.o.bacteria to create lichens, those remarkable confederacies which can achieve so much more than either partner on its own, and can produce body forms so dramatically different from the body form of either partner. Lichens (p.r.o.nounced LIE-kins) are sometimes mistaken for plants, and that isn't so far from the truth for plants too, as we shall see at the Great Historic Rendezvous, originally made a compact with photosynthetic micro-organisms for their food production. Lichens can loosely be thought of as plants-in-the-making, forged from two organisms. The fungus could almost be said to 'farm' captured crops of photosynthesisers. The metaphor gains from the fact that in some lichens the partners.h.i.+p is largely co-operative, and in others the fungus is more exploitative. Evolutionary theory predicts that the lichens in which the reproduction of the fungus and the photosynthesiser go hand-in-hand generally form co-operative relations.h.i.+ps. Lichens in which the fungus just captures available photosynthetic organisms from the environment are predicted to have more exploitative relations.h.i.+ps. And this seems to be the case.

What especially fascinates me about lichens is that their phenotypes (see the Beaver's Tale) look nothing like a fungus nor indeed like an alga. They const.i.tute a very special kind of 'extended phenotype', wrought of a collaboration of two sets of gene products. In my vision of life, explained in other books, such a collaboration is not in principle different from the collaboration of an organism's 'own' genes. We are all symbiotic colonies of genes genes co-operating to weave phenotypes about them.

Rendezvous 35 AMOEBOZOANS.

Joining us at Rendezvous 35 Rendezvous 35 is a little creature that once had the distinction of being, in the popular and even scientific imagination, the most primitive of all, little more than naked 'protoplasm': is a little creature that once had the distinction of being, in the popular and even scientific imagination, the most primitive of all, little more than naked 'protoplasm': Amoeba proteus Amoeba proteus. On this view, Rendezvous 35 Rendezvous 35 would be the final encounter of our long pilgrimage. Well, we still have a way to go, and would be the final encounter of our long pilgrimage. Well, we still have a way to go, and Amoeba Amoeba has, when compared to bacteria, quite an advanced, elaborate structure. It is also surprisingly large, visible to the naked eye. The giant amoeba has, when compared to bacteria, quite an advanced, elaborate structure. It is also surprisingly large, visible to the naked eye. The giant amoeba Pelomyxa pal.u.s.tris Pelomyxa pal.u.s.tris can be as much as half a centimetre across. can be as much as half a centimetre across.

Amoebas famously have no fixed shape hence the species name proteus proteus, after the Greek G.o.d who could change his form. They move by streaming their semi-liquid interior, either as a more or less coherent single blob, or by thrusting out pseudopodia. Sometimes they 'walk' on those temporarily extruded 'legs'. They eat by engulfing prey, throwing pseudopodia around it and enclosing it in a spherical bubble of water. Being engulfed by an amoeba would be a nightmarish experience, if you weren't too small to have nightmares. The spherical bubble or vacuole can be thought of as part of the outside world, lined by a portion of the amoeba's 'outer' wall. Once in the vacuole, the food is digested.

Some amoebas live inside animal guts. For example, Entamoeba coli Entamoeba coli is extremely common in the human colon. It is not to be confused with the (much smaller) bacterium is extremely common in the human colon. It is not to be confused with the (much smaller) bacterium Escherichia coli Escherichia coli on which it probably feeds. It is harmless to us, unlike its near relative on which it probably feeds. It is harmless to us, unlike its near relative Entamoeba histolytica Entamoeba histolytica, which destroys the cells lining the colon and causes amoebic dysentery, familiarly known (in British English) as Delhi Belly, or (in American English) as Montezuma's Revenge.

Three rather different groups of amoebozoans are called slime moulds because they have independently evolved similar habits (plus another unrelated group of 'slime moulds', the acrasids, which will join us at Rendezvous 37 Rendezvous 37). Of the amoebozoan ones, the best known are the cellular slime moulds or dictyostelids. They have been the life work of the distinguished American biologist J. T. Bonner, and what follows is largely drawn from his scientific memoir Life Cycles Life Cycles.

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Amoebas join. The word 'amoeba' is a description rather than a strict cla.s.sification because many unrelated eukaryotes exhibit an amoeboid form. The Amoebozoa include the cla.s.sic amoebas, such as the The word 'amoeba' is a description rather than a strict cla.s.sification because many unrelated eukaryotes exhibit an amoeboid form. The Amoebozoa include the cla.s.sic amoebas, such as the Amoeba proteus Amoeba proteus shown here, as well as most of the slime moulds about 5,000 known species in total. shown here, as well as most of the slime moulds about 5,000 known species in total.

Image: Amoeba proteus Amoeba proteus.

Cellular slime moulds are social amoebas. They literally blur the distinction between a social group of individuals and a single multicellular individual. In part of their life cycle, separate amoebas creep through the soil, feeding on bacteria and reproducing, as amoebas will, by dividing in two, feeding some more, then dividing again. Then, rather abruptly, the amoebas switch into 'social mode'. They converge on aggregation centres, from which chemical attractants radiate outwards. As more and more amoebas stream in on an attraction centre, the more attractive it becomes, because more of the beacon chemical is released. It is a bit like the way planets form from aggregating debris. The more debris acc.u.mulates in a given attraction centre, the more its gravitational attraction. So after a while, only a few attraction centres remain, and they become planets. Eventually the amoebas in each major attraction centre unite their bodies to form a single multicellular ma.s.s, which then elongates into a multicellular 'slug'. About a millimetre long, it even moves like a slug, with a definite front and back end, and is capable of steering in a coherent direction for example towards light. The amoebas have suppressed their individuality to forge a whole organism.

After crawling around for a while, the slug initiates the final phase of its life cycle, the erection of a mushroom-like 'fruiting body'. It begins the process by standing on its 'head' (the front end as defined by its crawling direction), which becomes the 'stalk' of the miniature mushroom. The inner core of the stalk becomes a hollow tube made of swollen cellulose carca.s.ses of dead cells. Now cells around the top of the tube pour into the tube like, in Bonner's simile, a fountain flowing in reverse. The result is that the tip of the stalk rises into the air, with the originally posterior end of the stalk at the top. Each of the amoebas in the originally posterior end now becomes a spore encased in a thick protective coat. Like the spores of a mushroom, they are now shed, each one bursting out of its coat a free-living, bacteria-devouring amoeba, and the life cycle begins again.

Bonner gives an eye-opening list of such social microbes multicellular bacteria, multicellular ciliates, multicellular flagellates and multicellular amoebas, including his beloved slime moulds. These creatures might represent instructive re-enactments (or pre-enactments) of our kind of metazoan multicellularity. But I suspect that they are all completely different, and the more fascinating because of it.

Rendezvous 36 PLANTS.

Rendezvous 36 is where we meet the true lords of life, the plants. Life could get along without animals and without fungi. But abolish the plants, and life would rapidly cease. Plants sit, indispensably, at the base the very foundation of nearly every food chain. They are the most noticeable creatures on our planet, the first living things any visiting Martian would remark. By far the largest single organisms that ever lived are plants, and an impressive percentage of the world's bioma.s.s is locked up in plants. This doesn't just happen to be so. Some such high proportion follows necessarily from the fact that almost1 all bioma.s.s comes ultimately from the sun via photosynthesis, most of it in green plants, and the transaction at every link of the food chain is only about 10 per cent efficient. The surface of the land is green because of plants, and the surface of the sea would be green too if its floating carpet of photosynthesisers were macroscopic plants instead of micro-organisms too small to reflect noticeable quant.i.ties of green light. It is as though plants are going out of their way to cover every square centimetre with green, leaving none uncovered. And that is pretty much what they are doing, for a very sensible reason. all bioma.s.s comes ultimately from the sun via photosynthesis, most of it in green plants, and the transaction at every link of the food chain is only about 10 per cent efficient. The surface of the land is green because of plants, and the surface of the sea would be green too if its floating carpet of photosynthesisers were macroscopic plants instead of micro-organisms too small to reflect noticeable quant.i.ties of green light. It is as though plants are going out of their way to cover every square centimetre with green, leaving none uncovered. And that is pretty much what they are doing, for a very sensible reason.

A finite number of photons reaches the planet's surface from the sun, and every last photon is precious. The total number of photons that can be garnered from its star by a planet is limited by its surface area, with the complication that only one side is facing its star at any one time. From a plant's point of view, a square centimetre of the Earth's surface that is anything but green amounts to a negligently wasted opportunity to sweep up photons. Leaves are solar panels, as flat as possible to maximise photons caught per unit expenditure. There is a premium on placing your leaves in such a position that they are not over-shadowed by other leaves, especially somebody else's leaves. This is why forest trees grow so tall. Tall trees that are not in a forest are out of place, probably because of human interference. It is a complete waste of effort to grow tall if you are the only tree around. It is much better to spread out sideways like gra.s.ses because that way you trap more photons per unit of effort put into growing. As for forests, it is no accident that they are so dark. Every photon that makes it to the ground represents failure on the part of the leaves above.

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Plants join. The plants comprise about 13 species of glaucophyte (single-celled algae, with chloroplasts whose morphology is very similar to free-living cyano-bacteria), 5,000 or so species of red alga, and about 30,000 species of 'green plant'. The green plants include many single-celled and colonial green algae, such as The plants comprise about 13 species of glaucophyte (single-celled algae, with chloroplasts whose morphology is very similar to free-living cyano-bacteria), 5,000 or so species of red alga, and about 30,000 species of 'green plant'. The green plants include many single-celled and colonial green algae, such as Volvox Volvox, as well as the more familiar mosses, ferns, conifers, flowering plants and the like. The order of branching of these three groups is reasonably well established, but the position of the plants in the eukaryotic phylogeny in general is disputed (see Rendezvous 37 Rendezvous 37).

Images, left to right: dulse ( dulse (Rhodymenia palmata); volvox (Volvox aurelia); giant sequoia (Sequoiadendron giganteum).

With few exceptions, such as Venus flytraps, plants don't move. With few exceptions, such as sponges, animals do. Why the difference? It must have to do with the fact that plants eat photons while animals (ultimately) eat plants. We need that 'ultimately', of course, because the plants are sometimes eaten at second or third hand, via animals eating other animals. But what is it about eating photons that makes it a good idea to sit still with roots in the ground? What is it about eating plants, as opposed to being a plant, that makes it a good idea to move? Well, I suppose given that plants stay still, animals have got to move in order to eat them. But why do plants stay still? Maybe it has something to do with the need to be rooted in order to suck nutrients out of the soil. Maybe there is too unbridgeable a distance between the best shape to be if you want to move (solid and compact), and the best shape to be if you want to expose yourself to lots of photons (high surface area, hence straggly and unwieldy). I'm not sure. Whatever the reason, of the three great groups of megalife that have evolved on this planet, two of them, the fungi and the plants, stay mostly still as statues, while the third group, the animals, do most of the scurrying about, most of the active go-getting. Plants even make use of animals to do their scurrying for them, and flowers, with their beauteous colours, shapes and scents, are the instruments of this manipulation.2 The pilgrims that we meet here at Rendezvous 36 Rendezvous 36 are not all green. The deepest divide are not all green. The deepest divide3 among them is between the red algae on the one hand and green plants (including green algae) on the other. Red algae are common on the seash.o.r.e. So are the various kinds of green algae, and green algae are also plentiful in freshwater. The most familiar seaweeds, however, are brown algae and these are more distantly related: they don't join us until among them is between the red algae on the one hand and green plants (including green algae) on the other. Red algae are common on the seash.o.r.e. So are the various kinds of green algae, and green algae are also plentiful in freshwater. The most familiar seaweeds, however, are brown algae and these are more distantly related: they don't join us until Rendezvous 37 Rendezvous 37. Of those whom we are greeting at the current rendezvous, the most familiar and the most impressive are the land plants. Plants conquered the land earlier than animals did. That is almost obvious, for without plants to eat, what would it profit an animal to be there? Plants probably didn't move directly from the sea onto the land but, like animals, went via freshwater. For an artist's impression of Concestor 36 see plate 46 see plate 46.

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If only Darwin and Hooker had had a computer. A tree of green plants, from the Deep Green program, A tree of green plants, from the Deep Green program, http://ucjeps.berkeley.edu/map2.html. The program runs on Mac or virus-compatible PC (enable Java in your browser). The root of the tree is at the bottom of the picture.

As usual, when we greet a large army of pilgrims, we find them already marching in complicated sub-groupings who have joined each other 'before' they rendezvous with us. In the case of the green plants, I strongly recommend a stunningly well-produced computer program called Deep Green, which, at the time of writing, is available on the internet. When you launch Deep Green, you see a rooted phylogenetic tree. Some of the branches have a name at the tip, the name of a plant or a group of plants. Some of them have no name and are pointing 'off the page'. The beauty of this program is that you may seize the tree with your mouse, and drag it, in the most delightfully natural and intuitive way, to see more of the tree. As you drag, you watch twigs sprout before your eyes, and as you swivel the tree round, you see a whole lot of new names appear on the screen, together with many new, unnamed branches. You then explore the tree as far as you like: it seems to go on for ever, which tells you what a huge diversity of green plants has evolved. As you climb through the branches, hand over hand as it seems, like a Darwinian monkey in evolutionary tree heaven, remember that each fork you encounter represents a true rendezvous point in exactly the sense of this book. It would be wonderful to have an animal version, too.

I ended a previous tale by remarking what delight it is to be a zoologist at such a time. I could have said the same about being a botanist. What a pleasure it would be to demonstrate Deep Green to Joseph Hooker in the company of his close friend Charles Darwin. I almost weep to think about it.

THE CAULIFLOWER'S TALE Written with Yan Wong The tales in this book are intended to be about more than the private concerns of the teller. Like Chaucer's, they are supposed to reflect upon life in general in his case human life, in our case life. What has the cauliflower to tell the huge a.s.semblage of pilgrims at the great get-together after Rendezvous 36 Rendezvous 36 when the plants join the animals? An important principle that applies to every plant and every animal. It could be seen as a continuation of the Handyman's Tale. when the plants join the animals? An important principle that applies to every plant and every animal. It could be seen as a continuation of the Handyman's Tale.

The Handyman's Tale was about brain size, and it made great play with the logarithmic way of making scatter plots to compare different species. Larger animals seemed to have proportionally smaller brains than small animals. More specifically, the slope of a log-log graph of body ma.s.s against brain ma.s.s was pretty much exactly . This fell, you will remember, between two intuitively comprehensible slopes:[image] (brain ma.s.s simply proportional to body ma.s.s) and (brain ma.s.s simply proportional to body ma.s.s) and[image] (brain area proportional to body ma.s.s). The observed slope for log brain ma.s.s against log body ma.s.s turned out to be not just vaguely higher than (brain area proportional to body ma.s.s). The observed slope for log brain ma.s.s against log body ma.s.s turned out to be not just vaguely higher than[image] and lower than and lower than[image] . It was exactly . Such precision of data seems to demand equal precision from theory. Can we think up some rationale for the slope? It isn't easy. . It was exactly . Such precision of data seems to demand equal precision from theory. Can we think up some rationale for the slope? It isn't easy.

To add to the problem, or perhaps give us a hint, biologists have long noticed that lots of other things besides brain size follow this precise relations.h.i.+p. In particular, the energy use of various organisms the metabolic rate follows a rule, and this was raised to the status of a natural law, Kleiber's Law, even though there was no known rationale for it. The graph opposite plots log metabolic rate against log body ma.s.s (the Handyman's Tale spells out the rationale for log-log plots).

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Law holds over twenty orders of magnitude. Kleiber's Law plot, adapted from West, Brown and Enquist [ Kleiber's Law plot, adapted from West, Brown and Enquist [304].