The Story of the Universe and Me: Where Did I Come From?
Part 5: Sex
Sexual reproduction has been a powerful force for the last 1.2 billion years. Exactly why and how it evolved remains unclear, but it does offer some advantages. It speeds up the spread of helpful traits through a population, creates new combinations of helpful genes (and removes damaged ones) and increases resistance to parasites. This all comes at the cost of a much slower and less certain reproduction rate. But for whatever reason, sex works, and it allowed those early eukaryotes to succeed and diversify.
Soon, such cells began to band together after splitting, instead of going their separate ways. This led to cellular colonies, working together to protect from predation. They were the first multicellular organisms. Sex became useful for producing new colonies of cells, while within those colonies the cells were still produced by splitting in half.
Eventually, cells within these organisms began to take on different roles. They specialised to form a harder skin on the outside while cells on the inside digested food, for example. All cells within an organism contained the same DNA, but expressed it differently depending on their roles.
This plurality of cells evolved independently many times, popping up in plants several times, three times for fungi, and once for animals. Yes, we’ve finally arrived at that point. Animals first evolved from flagellates, microorganisms able to propel themselves with the aid of little tentacle-like structures. This seems to have happened about 600 million years ago, and we haven’t looked back since. The earliest were sponges, but complexity steadily increased for another 58 million years. They developed muscles for pushing themselves through the water; nervous systems and, eventually, brains for responding quickly to stimuli; and sensory organs, sight, hearing, and smell, to go along with those nervous systems.
Then, diversity exploded. Oxygen concentrations had reached a new high, and the ozone layer was now in place to shield creatures from the sun’s ultraviolet rays. In harmony with a sudden surge in calcium levels in the oceans due to volcanic activity, from which animals could now build large skeletons, these circumstances allowed them to enter their element and take on a whole range of new and bizarre forms.
There were tiny shelled creatures of every shape; long, armoured arthropods crawling across the sea floor or swimming above it with their many articulated legs; radially symmetric echinoderms, the distant ancestors of starfish; large crustaceans, picking delicately at their food; and the first vertebrates, filter feeders with a head and a tail.
These developed into early fish, and obtained jaws; some evolved bony skeletons to replace their cartilage. Certain intrepid individuals, living in a freshwater environment, evolved strong, bony fins. Members of this group adapted further to shallow water and began to use these fins to walk along the bottom. As dissolved oxygen in shallow water was often lacking, some fish adapted to be able to breathe air as well as water, by drawing it into their swim bladder. These swim bladders increased in complexity, allowing oxygen to be consumed more efficiently, until they resembled the lungs of land animals. Walking was becoming a more important aspect of their lifestyle, and these fish evolved feet with which to propel themselves across the floors of their habitats.
Finally, about four hundred million years ago, in the brackish or freshwater swamps near the coasts of the Devonian Period, our first four-legged ancestor crawled from the water. An amphibian, he still would have spent most of his time underwater. Being cold-blooded, it is possible that he spent time on the water’s edge to bask in the sun. Some of his descendents slowly adapted to spending longer and longer stretches on land. Their legs got stronger, their ability to hear sounds from the air improved, and they developed mechanisms for preventing dehydration. Some remained amphibious, and would later become frogs and salamanders, while others grew to reject water completely and made a new home on land. They had evolved internal membranes for their eggs that allowed them to reproduce outside of the water. These were the first reptiles. They had emerged onto land to find an already thriving ecosystem, with invertebrates and plants having colonised the land a long time ago.
Making up for lost time, this new group diverged into sauropsids, from which all modern reptiles are descended, and synapsids. Synapsids evolved a faster metabolism and a more erect limb shape. For a time, a group of them known as therapsids were the dominant land vertebrates, filling a variety of ecological niches. Gradually, a group of them evolved the ability to generate their own heat through metabolism rather than relying on external sources as reptiles do. Predatory cynodonts also developed larger brains and possibly fur, but still would have laid eggs.
Unfortunately, 252 million years ago a mass extinction took place, wiping out seventy percent of all land animals. All ecosystems collapsed, and for 30 million years there was a great deal of instability as groups of animals competed for dominance. At last, archosaur reptiles emerged victorious over cynodonts as the dominant predators. From these archosaurs would spring dinosaurs, a group that reigned supreme over terrestrial environments for the next 135 million years. Cynodonts were forced into lesser roles. A subset of cynodonts found success, mostly as small insect-eaters living in the shadows of their reptilian brethren. They were mammals.
To be continued!