Best recipe for primordial soup

Back when whales walked on land and apes swam in the water

Richard Ellis's earlier books "Deep Atlantic" and "The Search for the Giant Squid" provided brilliant explorations of the ocean's depths and its mysterious life forms. His new book, "Aquagenesis," moves much further back in time. It's a dazzling tour de force that combines the deep insights of evolutionary biology, marine biology, and paleontology. Ellis traces the evolution of life in the sea from its earliest recorded appearance, dated by fossil evidence to about 565 million years ago, to the emergence of life from the sea about 55 million years ago.

The oldest fossils are simple animals shaped much like jellyfish or like annelids or worms, but the only characteristic that binds these fossils together is their symmetry. The most famous of these is Dickinsonia, which, in Ellis's words, "has the distinction of being the only fossil to be described as a jellyfish, a coral, a sea anemone, an annelid worm, an arthropod, a protozoan and a member of a new kingdom." These animals, who died out some 545 million years ago, were precursors to the greatest "evolutionary event in Earth's history: the Cambrian explosion."

The Cambrian seas were filled with "representatives of essentially all modern phyla." In these ancient waters, many marine invertebrates began to develop hard shells, passing along various forms and shapes to their descendants, such as the horseshoe crab and the nautilus.

Some of the largest marine invertebrates - the cephalopods, the order which includes octopi, squid, and cuttlefish - evolved during this period, eventually providing an important source of food for marine vertebrates.

According to Ellis, fish are one of the great evolutionary success stories: "Fishes have been swimming on Earth for more than 450 million years." They predated the dinosaurs by hundreds of millions of years, and they were the first creatures to have an internal skeleton. In that sense they are the ancestors of all vertebrates - amphibians, reptiles, birds, and mammals. Fishes are the most diverse of all vertebrates; more than 24,000 species inhabit the world's waters.

About 400 million years ago, aquatic vertebrates passed through a transitional stage from fishes to land reptiles. The fins of various species transformed into feet and legs, although the earliest animals of this kind probably used these limbs to walk on the bottom of the ocean shallows rather than on land. Amphibians evolved from the fishes, and reptiles from the amphibians.

A common misunderstanding of evolution is that it operates unidirectionally, moving from simplest to the most complex. Ellis demonstrates that many of the descendants of the marine ancestors who became terrestrial animals later returned to the sea. They "retrofitted" themselves for an aquatic life, "losing the legs their ancestors had worked so hard to acquire, forsaking modifications such as fur or external ears that made life on land possible."

The most startling examples are cetaceans, or whales. The oldest whale fossils are from 53 million years ago, and these early creatures descended from the mesonychids - omnivorous, hoofed land animals that spent part of their life in the water and part on land. Indeed, new fossil evidence confirms that whales descended from animals much like hippopotamuses.

In his last chapter, Ellis discusses Elaine Morgan's controversial theory that "humans are not descended from terrestrial hominids, like Australopithecus, but from aquatic apes." She uses characteristics such as bipedalism, hairlessness, a subcutaneous layer of fat, and voluntary breathing to show that penguins, seals, and walruses provide a clue to the ways that humans diverged from their primate ancestors to evolve in their own distinctive ways. Although Morgan's theory is not generally accepted, Ellis observes that "attention must be paid, if for no other reason than that many of her propositions make considerably more sense than the accepted ones."

Richly illustrated with black-and-white drawings, this detailed and insightful study is, quite simply, the best account we now have of the origins and evolution of life.

Henry L. Carrigan Jr. is a freelance writer living in Lancaster, Pa.

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