INFINITE LIFE

Most animals begin as eggs, and this is their story.

British science writer Howard, author of Death on Earth: Adventures in Evolution and Mortality, suggests that animals are simply vehicles to produce more eggs, and then he makes an entertaining case for that proposition. Life began nearly 4 billion years ago, but eggs came late. Until about 650 million years ago, tiny organisms often walled themselves off from a hostile environment in a mostly impermeable cyst, ready to reactivate when the good times came back. Beginning at this period their fossils began to reveal tiny collections of cells that resemble embryonic structures that will develop into new life. These were the first eggs, likely from ancient jellyfish and their kin. Working chronologically, Howard writes that by the Devonian Period, about 400 million years ago, “the egg had evolved a new place to hide in the ocean; no longer only found in pouches, or glued to rocks, the egg now occupied a new, secure space, inside the body of the female fish.” Many remained long enough to hatch, and this is not a modern development; live births are common in insects and ancient sharks. Land animals flourished well before the Permian Period (roughly 300 million years ago), which also saw the appearance of the amniotic egg that contained fluid-filled membranes surrounded by a hard cell to protect the embryo, allowing it to be laid on dry land. The egg reached its epitome in the mammal placenta, “the life-giving interface between mother and offspring,” allowing passage of energy and oxygen and extraction of waste but keeping what is essentially a foreign body separate from the mother’s immune system. Animal evolution is a snap compared to the minutia of animal physiology, but Howard has done his homework and delivers a painless but lucid education on a central feature of life.

High-quality natural history.