A damaged tapestry
Imagine what the world would be like if a major group of organisms became extinct. Let's say a massive environmental disaster wiped out a wide range of land animals, and none of the ungulates survived; that is all the hoofed mammals like cattle, deer, elephants and antelope, a diverse group including 220 living species.
Not only biodiversity would suffer. Life on Earth consists of a densely woven tapestry neatly integrating all living species, but also the planet's physical systems. Air and water quality and climate would be directly affected, even geological processes that shape the globe. As an example of animals that affect geology, sea creatures with shells bind atmospheric carbon (dissolved in water) into sodium carbonate on the sea floor, which gets subducteded through faults and becomes part of the earth's crust; this removes carbon from the atmosphere, cooling the planet.
A study of the emergence of life through geological time is bound to inspire awe in anyone. Dinosaurs alone--mythical monsters that really came to life--capture the imaginations of most children. Evolution has brought countless other species just as magnificent (if not in size), the vast majority of which have long since passed into soil, stone, air and water. They appear to have arisen in giant spasms of genetic creativity. To the casual student, these bursts look spontaneous.
In fact all these periods of diversification occurred because Earth in co-operation with previous life forms had set the stage. From the simplest microbes to Homo sapiens itself, every species has altered its environment. Some of these changes provided essential conditions for what came after, making the biosphere habitable for more complex organisms.
For example, new research suggests the appearance and spread of land plants around 700 million years ago, fixing carbon through photosynthesis and cooling the atmosphere, brought conditions favourable for the diversification of animal species during the Cambrian Period about 550 million years ago.
Another important change was brought on by the advent of herbivores with complex teeth. During the Devonian Period, between 408 and 360 million years ago, the first land animals emerged from the seas, including amphibians, insects and arachnids. But these slow, cold-blooded creatures weren't particularly successful at first because they couldn't digest plants very well. Early herbivorous amphibians and reptiles had teeth that could only tear leaves, the way an iguana does today. The rough herbage digested slowly in their stomachs. It was a relatively inefficient way of getting energy and the animals had low metabolic rates.
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