At The Waters Edge

At The Waters Edge, Macroevolution and the transformation of life, Carl Zimmer, Free Press (Simon & Schuster) ISBN 0-684-83490-1, 1998.

370 million years ancestral vertebrates migrated from the sea to the land; 55 million years the ancestors of the cetaceans migrated back from the land to the sea. This book tells the story of these two great movements across the water’s edge. The first half of the book follows life onto land; the second follows it back into the sea.

The term, macroevolution (evolution in the large), doesn’t have a sharply defined meaning which is universally agreed upon. The most common usage is to refer to the evolution of species and higher taxa. This usage is convenient because there is a natural division between evolution within species and the evolution of species. However it is also used to refer to the evolution of new features and capabilities. This usage is convenient for paleontologists who are necessarily looking at a broad, large scale picture. (Macroevolution in the latter sense is sometimes called mega-evolution. Of the making of neologisms there is no end.) Zimmer is concerned with macroevolution in the latter sense.

A few decades ago virtually nothing was known about either migration; there were speculations which relied much more on “could have been” than on substance. Since then much has learned. Quite a number of critical fossils have been. Much has been learned about how developmental processes (embryology) work and about what can be inferred about past development.

Zimmer covers a lot of ground in this book. He describes fossil finds and the people who found them. The evolution of blowholes and cetacean sonar systems, hox genes, cladograms, the mechanics of breathing, and the Arkansaw finds of Basilosaurus are all grist to his mill. It’s entertaining, informative, and well written. As a fault, though, it isn’t as coherent on the large scale as it could be. One thing that comes through quite clearly is that the actual course of events turns out to have unexpected twists and turns.

A standard early speculation about the migration to land was the ancestors lived in bodies of water (mud pools) that dried up. The ancestors developed lungs because the pools were often short on dissolved oxygen. They moved from pool to pool when pools dried up. (There are fish that do this now.) It was a good story. It was wrong.

Lungs, it turns out, were standard equipment in both ray-finned (teleosts) and lobe-finned fish some 60 million years before the migration to land. Air breathing is a useful adjunct for a fish – if there are no hungry birds in the sky. Lungs were an adapatation for being a better fish in the open sea long before the migration to land was made.

The ancestral tetrapods were bottom feeders (some living fossils of this type can be found in the Senate). The lobe fins of the lobefish evolved for bottom walking through weedy marshes and lagoons. (It’s hard to swim through thick weeds.) Fingers (toes if you prefer) developed next and the lobes strengthened into limbs. All of this happened underwater – the ancestral tetrapod was a bottom feeder deluxe. The early tetrapods had eight toes rather than five. S.J. Gould entitled one of essays about Acanathosega and company, Eight Little Piggies.

Oddly enough, we still don’t know why they went onto land. It probably wasn’t for food – the ancestral tetrapods weren’t well adapated for feeding on insects. One speculation is that they went ashore to lay eggs. Eggs of other animals are a very popular food source. In those days laying eggs on land, buried in the soil, would have been safe. It’s a theory; it may even be right but it’s only a theory. We don’t really know.

As a small point, there is a traditional sequence of land vertebrates, amphibians, reptiles, and then mammals and birds. The sequence is wrong. Amphibians and amniotes are two separate branches. The reptiles, mammals, and birds are all amniotes, i.e., they have eggs in a sac. The different branches of amniotes are all very ancient; the history isn’t one of a linear sequence.

The second half of the book focuses on the cetaceans and where they came from. There are three great branches of the amniote tree, the synapsids (they became mammals), the diapsids (crocodiles, lizards, dinosaurs, and birds) and the parareptiles (turtles). For 160 million years the diapsid dinosaurs were the kings of creation. Then, 65 million years, they met der tag. During the succeeding 10 million years the little nocturnal mammals spread throughout the world, grew greatly in size, and filled the vacated niches with a variety of forms. Among the new masters of the land were a strange group of animals called mesonychids.

Pachyaena was a representative mesonychid insofar as there is such a thing as a representative mesonyschid. It is a puzzling animal. It was sort of built like a hyena with hooves. It had an outsized head with wide powerful jaws and a rigid backbone. It was built for running. It was a meat eater but not a true carnivore; whatever it ate must have been hard on the teeth which are very worn. One possibility is that it included shellfish and turtles in its diet. Maybe it was ancestral to the whales; maybe not.

In the Eocene India was well on its way to crunching into Asia. The Tethys sea between them was slowly disappearing. Along the shores of the disappearing Tethys was an animal called Ambulocetus (“walking whale”).

Its four-hundred-pound body – an enormous crocodile like head, a wide chest, and a long tail – sat on squat legs. It still had the tall projections rising from its neck vertebrae that mesonychids had used to hold up their heavy heads. The width of its chest pushed its hands out to either side like seal flippers, and the giant feet on its crouched hind legs slapped awkwardly on the ground. Ambulocetus could shamble on land if it had to, but the shape of its spine told Thewissen where its gifts lay. It had lost the locking tabs that kept mesonychid spines rigid, and its general geometry was closer to an otter’s than an other animal’s.

In short, Ambulocetus was built very much like a hairy crocodile. If the artists illustration is at all accurate, meeting it would have been hairy indeed. The story of how the descendents of Ambulocetus, Rodhocetus, Basilosaurus, and company, took to the ocean is a complicated one. The last third of the book is devoted to that story.

Among the enthusiastic warbling on the dust jacket (said warblings being warranted) is one from John Horgan (End of Science):

“Richard Dawkins and Stephen Jay Gould, move aside. Carl Zimmer brings evolutionary biology to life more vividly than any author in recent memory. This is marvelous book by one of our best young science writers. Carl has persuaded me that evolutionary biology remains a vital, even thrilling, field.”

Given the endorsee, one could scarcely ask for a more glowing endorsement. It is charming to see something pierce Horgan’s self imposed fog.