Lesson 5: Life's History: The Great Adventure 5.0 Introduction and Overview The surface of Earth, with few exceptions, teems with life. In every drop of seawater, in every scoop of wet soil, millions of microscopically small cells go about the business of life: growing and reproducing. Larger organisms, with many-celled bodies live throughout the oceans, lakes and rivers of the world, and on solid ground. They are intimately tied to the world of microbes, depending on it for sustenance as well as fighting off a take-over of their own bodies by microbes. Green forests and grasslands mark the face of the land where there is sufficient rain. In the ocean, the green regions are where light-gathering microbes have access to phosphate and iron. The diversity of life is truly astonishing. There are millions of different kinds of organisms. Where do they all come from? The standard answer given by modern science to this question is well known: All living organisms, including archea, bacteria, protists, plants, fungi and animals, arose from a few primitive ancestors or (more likely) from a single ancestral cell, through evolution. In biology, the concept of evolution is fundamental. We know that evolution works from the fact that we humans have generated domesticated plants and animals from wild stock. The corn we grow is in all respects similar to the wild variety, except that it makes nice large kernels. We humans bred it that way, by always selecting the plants with the biggest kernels for producing seed for the next generation. If we agree that every life form has to have a parent, then there is no escaping evolution, even across major categories. The reason is that rocks more than a billion years old only have fossils from single-celled organisms. Modern-looking many-celled organisms are restricted to the last 600 million years or so. They too had to have ancestors. There weren't any life forms except microbes from which to draw ancestors, if we go far enough back. If we agree that every human being that ever lived, and all its ancestors, had to have parents, then we must admit a microbes-to-man history, with intermediate stages resembling fish, reptiles and monkeys. The only way to get out of this logical straightjacket is to claim that some "original" people had no ancestors. Such a claim was routinely made up to 150 years ago. However, scientists no longer take this claim seriously. Spontaneous generation has been out for some time. Humans resemble many other mammals, especially primates, in great detail, bone for bone. If we accept a no-ancestor hypothesis for people, we have to erect a biological wall between people and other primates, and this would destroy our ability to explain the striking similarities in anatomy and genetic endowment across the entire group. But are we not made from dust on the ground, at the end of creation? Of course we are. The elements in our bodies, (carbon, hydrogen, oxygen, nitrogen, ron, sulfur, phosphorus and many others), are precisely those found in the dust of the Earth. And, indeed, we (and the other higher primates) only appeared rather recently in Earth history. Evolution takes time. In fact, without the concept of evolution, nothing in biology makes much sense. We would be completely at a loss to explain why organisms can be classified into different groups whose members show great similarity in shape, in behavior, in life history, and in their genetic code. Without the concept of evolution, we have no entry to the history of Life. Life becomes a phenomenon without any history whatever. In truth, of course, life does have a history. The reconstruction of this history is one of the great adventures of the human mind. Adventures are not for everyone that is why so many, sadly, have been unable to participate in the excitement. Here we briefly trace major stages in this adventure, the great drive toward understanding what Life is all about, based on a belief that it is possible to find out the truth about what really happened.