. . . along with some ribs, vertebrae and shoulder bones. But it's the skull that captures the most attention. The multitude of variations that occurred as animals moved in delicate steps from water onto land that make the story most interesting. And Jenny Clack's story of our four-legged forebears is a wondrous tale. Ever since Charles Darwin explained the nature of life's evolution, the question of how sea creatures moved to the land has been an enigma. Consider the many issues involved: walking, breathing air instead of filtering water, hearing in air instead of water, how to feed - and where, and protecting eggs. Clack shows how these topics were addressed by slow, incremental changes in body plan, with changes in one area integrated with those in another.
Walking on land meant not only building bones strong enough to support the body, but muscles to drive them. The humerus, the single bone in your upper arm, not only had to be stronger, it had to have joints for a new form of movement. A stride is far different from the flapping of a fin, so the paddling fin had to change. Clack discounts the older, simpler views that the "lobe-finned" fish just developed better "legs". Moving from the sea requires more than just crawling up the beach. There had to be an intermediate step. Clack finds that step in brackish lagoons and shallow, meandering rivers. There, the new four-legged creatures learned to walk on silty soils and learn to mix air and water breathing methods.
It was a reinforcing cycle as the change in surroundings developed new capacities. Diet went from fish to insects. No longer able to simply swallow prey as fish do, tetrapods began feeding on insects and their own smaller cousins. That meant biting and chewing, requiring stronger jaws and specialised teeth. Skulls once short and narrow became wide and flat. This reorganising of the entire skull required new musclature for support. The more time on land, Clack shows, meant not only stronger legs, but a sturdier backbone. Ribs developed that held muscles for breathing. Although the earliest tetrapods likely gulped air as a fish gulps water, before long they were using their nostrils to fill lungs.
As should be obvious, this isn't a simple narrative. The fossil bones are meticulously detailed - when they are available. Clack's task is rendered more difficult by the paucity of fossils. She has been lucky in her own finds in Greenland and Scotland. Others have encountered Carboniferous fossils in the Ohio Valley, Nova Scotia and Australia. The real treasures should be in coal seams where plant remains have become burnable stone. However, mining operations leave little opportunity for discovery. What has been found has often been misinterpreted. In order to depict what happened to tetrapod bodies over time, she is meticulous in describing individual bone types and how they changed. She helps the description with photographs and a wealth of line drawings. Still, this isn't a book for the uninitiated. It requires careful reading and no little back-flipping of the pages. The endeavour is well worth the effort, however. Clack has established an new foundation for understanding where and how creatures like ourselves originated. [stephen a. haines - Ottawa, Canada]
