Summary of Our Improbable Universe
Chapter 7: Bio Big Bang
Until seven hundred million years ago the only multicellular organisms were simple filaments and sheets of indistinguishable cells. The concentration of oxygen was too low to allow the cells on the interior of a complex organism to get oxygen by diffusion. But the continued build up of oxygen in the atmosphere eventually made it possible for the interior cells to breath. At this point the evolution of multicellular organisms took off in a big way.
The first complex animals to evolve were sponges, jelly fish and worms. They grazed on the abundant algae and single celled animals of the day. After a few hundred million years some of these animals evolved into predators that lived off of other complex animals. This step in evolution may have been the motivation for the greatest outburst of biological creativity in the history of life on Earth. During the Cambrian explosion the basic body plans for all of the complex animals on Earth were evolved in the space of only ten million years. There are numerous hypotheses to explain why this suddenly happen half a billion years ago. The recent discovery of a nightmarish creature, that was at the top of the food chain of the time, may explain some of the story. The rasp like teeth in its circular mouth explain the sudden appearance of hard shelled creatures.
Mobility is always a very effective way of avoiding being eaten. Our Cambrian ancestors were mollusks that lived on rocks as adults. They released eggs into the water that hatched into free swimming larva. When they matured they would attach to rocks and continue the cycle. The scarcity of real estate and the vulnerability to predation of a sedentary life style led to the next key step in our evolution. Our ancestors chose to not grow up. They remained immature and free swimming for their entire life. They evolved into the fish that dominate the sea to this day.
While all of this creativity was going on in the ocean there was very little that was happening on the land. Only simple mosses and lichen could be found there. But gradually the crab like creatures that inhabited the tidal pools of the Cambrian era evolved the ability to breath when the tide was out and they were trapped on land. They slowly evolved into insects and other land loving anthropoids. For two hundred million years they had free reign of the land. But their method of breathing and their hard exterior skeleton limited their size. Given enough time, they might have evolved the kind of intelligence that is the basis for our civilization. But the slow rate of evolution that characterizes these creatures speaks against this speculation.
The true inheritors of the Earth evolved from an amphibian like fish that first crawled out of the water a third of a billion years ago. One of its motivations for doing this was to eat insects. Another was to avoid the giant fish that inhabited the inland streams and pools of the day. A third was to find a new pool that hadn't dried out yet. As its life style became more terrestrial, it evolved better lungs, skin, and legs. The adaptation that freed it from its dependence on pools of water for good was a leathery covering for its eggs. These eggs could be lain on land were pools never formed and where aquatic predators never go. This freedom from the water is the primary distinction between reptiles and amphibians.
Giant amphibians and reptiles dominated the land for a hundred million years before the next major evolutionary adaptations came about. These adaptations were motivated by the opening of new biological niches in the wake of the worst environmental disaster of the last billion years. One theory (of many) is that a quarter of a billion years ago a plume of molten rock from deep within the Earth broke the surface in Siberia. A flood of lava a thousand miles in diameter covered the land. The ash and sulfurous gases that were released blocked out enough sun light to precipitate one of the deepest ice ages of all time. Ninety six percent of all species became extinct. It was the worst mass extinction event ever. In its aftermath, a line of reptiles gradually refined the warm blooded adaptations that characterize mammals. Twenty million years later another line of reptiles evolved into the dinosaurs.
The dinosaurs and the mammal like reptiles evolved into opposite biological niches. The dinosaurs became huge and dominated the day. The ancestors of the mammals became tiny and came out at night. This is how it would be to this day if it were not for the greatest meteor impact anywhere in the inner solar system in the last billion years. A meteor that was more than six miles in diameter crashed into coast of the Yucatan Peninsula in central America sixty five million years ago. It made a crater that is over a hundred and fifty miles in diameter. A thousand foot high wave deposited ocean floor debris in the mountains of Cuba and Haiti. Global forest fires were set off by the rain of molten rock. The sun was shut out for more than a year by the ash and dust. In the extended winter that followed all surface animals that were larger than fifty pounds went extinct. All lines of the dinosaurs, except for the birds, were wiped out completely.
The enrichment of oxygen in the atmosphere allowed the evolution of complex animals over half a billion years ago. After that, numerous nightmarish events flogged them up the evolutionary ladder. The evolution of predators, excess volcanism, and a horrific meteor seemed like Yin (darkness and trouble) at the time. The Yang that resulted was the evolution of more complex and adaptive creatures that could cope with these disasters.