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August 21 - October 11, 2022
Closely related to us gnathostomes, placoderms were the earliest fish to have hinged jaws, and simplified teeth to fill them.
It’s not just lower jaws that trace back to placoderms. Our legs originate in the paired pelvic fins that are first found in early placoderms.
The inference that these ancient fish bore live young has been cemented by the recent discovery of a fossil placoderm containing an embryonic baby: the earliest pregnant mother in the fossil record.
Why did both the jawed placoderms and the jawless ostracoderms develop such heavily fortified bodies? What was it about those Palaeozoic seas that demanded such formidable protection? The presumed answer is equally formidable predators, and the obvious candidates, apart from other placoderms, are the eurypterids or sea scorpions, some more than two metres in length—the largest arthropods that ever lived.
Finally, we turn to the only jawless fish to have survived to the present. The eel-shaped lampreys and hagfish are more distantly related to us than the plated fish of the Palaeozoic.
The vertebrates hark back to the deep Cambrian.
Rendezvous 22 is a major milestone. From now on, for the first time, all the vertebrates are united in a single pilgrim band. It is a big event because, traditionally, animals were divided into two major groups, the vertebrates and the invertebrates.
Important though we vertebrates think ourselves, we don’t constitute even a whole phylum. We are a subphylum of the phylum Chordata, and the phylum Chordata should be thought of as on a par with, say, the phylum Mollusca (snails, limpets, squids, etc.) or the phylum Echinodermata (starfish, sea urchins, etc.).
The phylum Chordata includes creatures that, while not vertebrates, still possess a notochord
there is a massive difference in genome complexity between (all) vertebrates and (all) invertebrates: ‘It is, at the genetic level, perhaps the biggest change in our metazoan* ancestry.’
Human adult haemoglobin is actually a composite of four protein chains called globins, knotted around each other like an orgy of hagfish to form a beautifully functioning parcel. The sequence of amino acids in the chains shows that the four globins are closely related to each other, but they are not identical. Two of them are called alpha globins (each a chain of 141 amino acids), and two are beta globins (each a chain of 146 amino acids). These globins are encoded by genes on different human chromosomes: alpha on chromosome 11, beta on chromosome 16.
The detailed similarities between globin genes reveal that they cannot have arisen independently of each other.
They are parallel copies, what a geneticist would call ‘paralogous genes’.
On the scale of whole organisms, of course, all vertebrates are cousins of each other too. The tree of vertebrate evolution is the family tree we are all familiar with, its branch-points representing speciation events—the splitting of species into daughter species. In reverse, they are the rendezvous points that punctuate this pilgrimage. But there is another family tree occupying the same timescale, whose branches represent not speciation events but gene duplication events within genomes.
And the branching pattern of the globin tree looks very different from the branching pattern of the family tree, if we trace it in the usual, orthodox way, with species branching to form daughter species.
Every gene has its own tree, its own chronicle of splits, its own catalogue of close and distant cousins.
The DNA sequences which produce our haemoglobin, the alpha and beta genes, started out as sisters. About half a billion years ago, in an early fish (perhaps of the ostracoderm or placoderm grade that we described previously), an ancestral globin gene accidentally duplicated, both copies remaining in that fish’s genome. One copy was destined to give rise to the alphas, on what would eventually become chromosome 11 in our genome, the other to the betas, now on our chromosome 16.
Given that the duplication which produced alpha and beta versions took place half a billion years ago, it will of course not be just our human genomes that show the split, and possess both alpha genes and beta genes in different parts of our genomes. We should see the same within-individual split if we look at the genomes of any other mammals, at birds, reptiles, amphibians or bony fish—for our common ancestor with all of them lived less than 500 million years ago.
When you and I were foetuses in our mothers’ wombs, we needed oxygen, which we extracted from our mothers’ blood via the placenta. This requires a special type of foetal haemoglobin, which binds oxygen more tightly, and hence can grab it from the adult haemoglobin of the mother. Foetal haemoglobin is made by replacing the two adult beta globins with two gamma globins. In this case, gene duplication and subsequent specialisation provided a necessary step on the path that culminated in the placental mammals.
Lampreys and hagfish are the most distantly related vertebrates of all, the only ones who diverged more than half a billion years ago.
the vertebrate-only globins, which presumably arose between Concestor 22 and Concestor 23, fall into four major lineages. These are represented by haemoglobin (alpha and beta), myoglobin, cytoglobin (also present in lampreys), and globin Y.
vertebrates, DNA sequences of all sorts show duplication at the base of the vertebrate tree (the classic example is a fourfold duplication of the Hox gene cluster, the subject of the Fruit Fly’s Tale).
On one momentous day, more than half a billion years ago, many, perhaps all, of our ancestor’s chromosomes were copied, and on a later day in a descendant of that ancestor those copies themselves were recopied, to produce a genome containing four instances of most genes.
the vertebrate genome as being massively more complex than the genomes of their nearest invertebrate relatives.
Massive duplication and divergence of DNA may mark the start of the rise of the vertebrates.
Our gene divergences go back much further in time too, predating the vertebrates, predating the animals, even predating the split between animals and plants, back to our time as single-celled bacteria, and to the origin of life itself. Each one of our genes, if you go ...
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sea squirt is something else. It doesn’t swim like a fish. It doesn’t swim like anything. It doesn’t swim. It is far from clear why it deserves the illustrious name of chordate at all. A
typical sea squirt is a bag filled with sea water, plus a gut and reproductive organs, anchored to a rock.
The bag is topped by two siphons—one for drawing water in, the ot...
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All the pilgrims we have met belong to a single great phylum, the Chordata, which is rounded off by a tidy little animal who wriggles up to join us here. It is the lancelet or amphioxus.
It is a protochordate, not a vertebrate, but it is clearly related to the vertebrates, and placed with them in the phylum Chordata.
The 32 known species of fish-like animals commonly known as lancelets are known to be the last chordate group to join our pilgrimage.
We have no fossil of the common ancestor we share with the chimpanzees, even though that was less than 10 million years ago. But we were able to guess, with misgivings, that the ancestor was most likely to have been, in Darwin’s famous words, a hairy quadruped, because we are the only ape that walks on its hind legs and has bare skin.
Our pilgrimage is now a milling horde, having amassed all the vertebrates, together with their primitive chordate cousins, amphioxus and the sea squirts. It comes as quite a surprise that the next pilgrims to join us, our closest relatives among the invertebrates, include those strange creatures—we shall soon refer to them as ‘Martians’—the starfish, sea urchins, brittle stars and sea cucumbers.
These, together with a largely extinct group called the crinoids or sea lilies, comprise the phylum Echinodermata, the spiny-skinned ones.
super-phylum called Ambulacraria.
Earth’s animals are mainly bilaterally symmetrical: they have a front end and a rear end, a left side and a right side. Starfish are radially symmetrical, with the mouth right in the middle of the lower surface, and the anus right in the middle of the top surface.
If ‘Martian’ starfish have sides at all, they have five sides (or, in a few cases, some larger number), not two like most of the rest of us on Earth. Earth’s animals mostly have blood. Starfish have piped sea water instead.
‘Forward’ is arbitrary for these ‘Martians’, and that includes brittle stars and most urchins as well as starfish. Unlike most Earth life forms, who have a definite front end with a head, a starfish can ‘lead’ with any one of its five arms.
Less than 4 per cent of all animal species are closer cousins to us than starfish are. By far the greater part of the animal kingdom is yet to join our pilgrimage. And they mostly arrive all together, at Rendezvous 26, in one gigantic influx of pilgrims.
The protostomes are about to overwhelm even the multitude of pilgrims who are already on the march.
the greater part of the animal kingdom is deeply divided into two great subkingdoms, the Deuterostomia and the Protostomia.
Animals typically pass through a watershed event in their early life called gastrulation.
Gastrulation is something that all animals do early in their life. Typically, before gastrulation, an animal embryo consists of a hollow ball of cells, the blastula, whose wall is one cell thick. During gastrulation the ball indents to form a cup with two layers.
The opening of the cup closes in to form a small hole called the blastopore.
Protostome means ‘mouth first’. Deuterostome means ‘mouth second’.
There are indeed two main kinds of animal, the deuterostomes (our lot) and the protostomes (them over there).
However, some phyla that used to be included in the deuterostomes have now been moved by molecular revisionists, whom we shall follow, to the protostomes. These are the three so-called lophophorate phyla—the phoronids, brachiopods and bryozoans—now grouped together with the molluscs and annelid worms in the ‘Lophotrochozoa’ division of the protostomes.
There are also some animals that don’t belong to either the protostomes or the deuterostomes, but we’ll meet them at later rendezvous.
Rendezvous 26 is the biggest of all, more of a gigantic rally of pilgrims than a rendezvous. When does it happen? Such ancient dates are hard to estimate. Our attempt of 560 million years is plus or minus a large margin of error, and probably more plus than minus in this case.
