Your Body - The Fish That Evolved. Dr. Keith Harrison

       The human pedigree

       T he story of our bodies did not begin when our ape-like ancestors left the trees. By then, it was already a long tale stretching back before the evolution of the first fish, 500 million years ago. We are descended from those fish, as is every other animal with a backbone that has ever lived, from the smallest frogs and lizards to the largest elephants and dinosaurs.

      Once fish appeared in the ancient seas, they became increasingly widespread. Some moved into fresh water, then onto the land. Natural selection did its work and the first amphibians evolved. Some amphibians turned into the first dry-land vertebrates, the reptiles, and, while one group of reptiles evolved larger and larger bodies and became the dinosaurs, others became the first mammals and grew smaller and smaller. When the dinosaurs faded, leaving only their descendants the birds to dominate the air, mammals took over the ground and the trees. Eventually, one group of mammals stood upright and walked out of the forest. The rest, as they say, is history.

      This is the story of what created that history. We will explore our time as fish and will track evolution through our amphibian and reptile pasts to our life as mammals. Each stage of this journey has left its mark on our bodies and to understand why we look the way we do today we must first understand where we came from.

      As vertebrates, we can trace many important parts of our design back to the first fish, but our overall shape is even older than that.

      Five hundred million years ago, the seas were teeming with animals but every one was a type of invertebrate. Today, we are familiar with many of their relatives: insects, arachnids and crustaceans (with bodies encased in hard jointed shells); molluscs (including clams with two hinged shells, snails with a spiral shell, garden slugs and squid with an internal shell, octopus with no shells); echinoderms, well named for their ‘prickly skin’ (starfish, sea-urchins, sea-cucumbers); segmented worms and their relatives (earthworms, rag-worms, lugworms, leeches); unsegmented roundworms and flatworms; sea-anemones, corals, jelly-fish; and other groups less well known and too numerous to list.

      In these ancient seas filled with invertebrates, an innovation appeared that was to change the face of nature forever. A species evolved a stiffening rod down the centre of its body. It had become a fish. This rod would later be turned by natural selection into a row of bones, the vertebrae, and we vertebrates had started our epic journey. Scientists still don’t know which invertebrate group we have to thank for our backbone, a structure so dominant in our bodies and in our minds that we refer to it in the singular even though it has more than 26 separate bones, and which we cite as the epitome of strength: ‘Show some backbone. Are you spineless?’ However, we can say something about our invertebrate ancestor’s body.

      What we inherit from our invertebrate ancestors

      Animal bodies can take various forms. Some radiate outwards in all directions from the centre like a starfish or a coral polyp, but most have sides that are mirror images of each other. Whatever they have on one side they also have on the other side, and many of the organs occur as pairs. Those parts of the anatomy there is only one of, like the intestine, usually lie along the mid-line.

      The invertebrate that became a fish was one of these bilaterally symmetrical forms. Every vertebrate that has ever lived has consequently followed the same pattern, including us. We have paired arms and legs, eyes, ears, nostrils, lungs, kidneys, ovaries and testes (testicles), and in the centre-line of our bodies we have one brain (with some paired aspects), one spine, one heart (which leans to the left), one reproductive organ and one intestine (greatly coiled so it can be up to six times longer than we are) with its one entry and one exit.

      Our early invertebrate ancestors were apparently animals that moved through their environment as we also inherited from them a head and a tail, although since we stood upright these have become the top and the bottom. Any animal that moves – whether it’s a worm, a lobster or a snail – has evolved sense organs at its front end, the end that encounters the environment first. Having all your sense organs on your tail would not have much survival value. An animal needs to know that it’s about to crawl into a predator’s mouth not that it’s just finished crawling into a predator’s mouth. For a similar reason an animal’s mouth is usually at the front of the body so it’s the first thing to encounter food. This is especially important for predators where the food may be able to escape if it gets some warning (lions wouldn’t get many dinners if they backed into zebras tail-first).

      This sensible arrangement has led in almost all animal groups to the evolution of a head, which we wear as a curiously shaped stalked ball balanced on the top of our torso, but which other animals have