Old Fourlegs Revisited

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Last week the world press took note of a fish hauled up off the coast of Zanzibar. (AP, Reuters). Why did they care? Because the animal was one of the most celebrated fish of the sea: it was a coelacanth.

The coelacanth is an ugly, bucket-mouthed creature. At first scientists only knew it from its fossils, the youngest of which was 70 million years old. In 1938, however, a flesh-and-blood coelacanth was dredged up near East London, South Africa. The five-foot long beast had many of the hallmarks of fossil coelacanths, such as hollow spines in their vertebrae, peculiar lobe-shaped fins, and a joint dividing its eye and “nose” from its brain and ears. The coelacanth became a celebrity in the, hailed as a “living fossil.”

Its fame was reinforced by its elusiveness. It was not until 1952 that a biologist found a second coelacanth, caught this time off the Comoros Islands. Scientists chased the coelacanth so doggedly in part because of what it might reveal about ourselves. Fossils of the coelacanth lineage dated back over 300 million years to the Devonian Period. They belonged to the same group of fishes as our own ancestors (known now as lobe-fins). While the ancestors of coelacanths stayed in the water, our own fishy ancestors climbed on land and evolved into mammals, birds, reptiles, and amphibians. (See my book At the Water’s Edge for more on this transition.)

Since the living coelacanths seemed to resemble Devonian fossils of coelacanths, the term “living fossil” seemed to carry a double meaning–a lineage that was not just lost for a long time, but also had changed very little. Scientists became particularly intrigued in the fins of the coelacanths, looking at them as potential models for what the fins our own ancestors looked like. One book on coelacanths was even entitled “Old Fourlegs.”

Scientists have learned a lot about coelacanths since then. They’ve discovered the fish not just in Zanzibar, but all the way across the Indian Ocean, in Indonesia. They’ve filmed it underwater. They’ve analyzed its genome and found new coelacanth-related fossils. The fish remains fascinating. Its DNA confirms what paleontologists saw in fossils: Of the 20,000 species of fish, coelacanths (as well as their fellow lobe-fins, the lungfish), are our closest living relatives.

But these days coelacanths don’t wear the label “living fossil” very well any more. In some ways, it turns out, it is we–not the coelacanths–that are the living fossils.

I wrote about one example of this table-turning last year in an essay for PLOS Computational Biology. Land vertebrates and coelacanths share a piece of virus-like DNA that infected a common ancestor long ago. In our lineage, that DNA became frozen in place, while in the coelacanth it continued to make new copies of itself that were reinserted in the genome. (For more details, read my essay here.)

This week, in the wake of the Zanzibar discovery, a new example has come to light. Instead of virus-like DNA, this example involves our hands and their fins.

The pectoral fin of living coelacanths (corresponding to our arm) is composed of a long chain of bones stretching out from its body. At the end of that chain (sometimes called an axis) is a symmetrical fan of smaller bones known as radials. The radials and axis are encased in a fleshy fin. Lungfish, the other living group of lobe-fin fish, have an axis made of a long chain of bones, with a symmetrical set of radials.

This similarity led some scientists to propose that our own fishy ancestors had fins with a similar anatomy. Only later did our own lineage evolve a different plan. Instead of a symmetrical set of radials, our radials began to branch off of one side. Eventually, those radials evolved into fingers and other parts of the arm. (To illustrate all this, I’ve cribbed a diagram from a new paper below. Latimeria are living coelacanths, Glyptoliepis and Neoceratodus are lungfish. Our own lineage is represented here by Tiktaalik, a Devonian lobe-fin that had evolved some of the key features of land vertebrates.)

Paleontologists have been digging up coelacanth fossils for many decades, but until now they have not found good remains of an early coelacanth fin. In the latest issue of Evolution and Development scientists at the University of Chicago describe a fin fossil they found in Wyoming. They dubbed the 360-million year old fish Shoshonia arctoperyx. While they only found part of the Shoshonia fin, it was enough for them to draw some surprising conclusions. Most surprisingly of all is that the fins of living coelacanths are probably different in some important ways from the fins of the first coelacanths.

As the illustration below demonstrates, Shoshonia had the same one-sided anatomy that we have in our arms. When the Chicago scientists drew a tree of lobe fins and noted the evolutionary steps along each branch, they found that the common ancestor of all lobe fins probably had this one-sided anatomy. Our own ancestors retained a primitive one-sided plan, which was elaborated into hands and feet. Only later did lungfish and the ancestors of living coelacanths evolve more symmetrical fins, along parallel evolutionary tracks.

To borrow from Pogo, we have met Old Fourlegs, and it is us.

[For more on our primitive hands, see this post from May on the development of paddlefish embryos.]

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