August 26, 2009 11:59:00 AM
It is one of the shibboleths of evolution that the blind forces which change genes and change creatures have no aim or direction. Our hands and the wings of bats may be wonderfully engineered biological machines, and may arise from the same basic limb design, but it is wrong to think that evolutionary forces set out to build up progressively so that hands and wings could emerge with their current efficient designs.
It is hard, however, to get away from the idea of life forms progressing or ascending; we do, of course, speak of “lower life forms” without thinking of how astonishingly complex even an amoeba is. Nick Lane, a biochemist, knows that we are not evolutionarily “climbing the ladder of life”, and especially knows that it is a parochial view that puts humans at the uppermost reaches of the biological tree.
Nonetheless, his most recent book is called “Life Ascending: The Ten Great Inventions of Evolution” (Norton). These 10 inventions are steps, if not steps up, in the complexity of life. The subtitle of his book also bears examination, and he knows it. In his introduction, he writes, “Evolution has no foresight, and does not plan for the future. There is no inventor, no intelligent design. Nonetheless, natural selection subjects all traits to the most exacting tests, and the best designs win out.” Each of the chapters here looks at one of the 10 winners within those tests.
Lane admits he has made a subjective “10 Best List”, but he did have criteria. Each invention had to revolutionize the world, be of surpassing importance now, be due to evolution by natural selection rather than due to cultural forces, and had to be iconic in some way. What is significant in his book is that, as befits a biochemist, he has not concentrated on, say, hand and wing morphology, but on the molecules within cells that make the whole biological and evolutionary process go.
To start off, necessarily, Lane considers the origin of life. Darwin put forward his view of life arising from just the right chemicals happening to be held in a pond at just the right temperature. Our best understanding now is that the warm pond probably isn’t the right place to look for the origin. A pond of chemicals is too bland to start molecules reacting with each other. Darwin had no way of knowing about the current best candidate for primordial life, the fissures within the basalt of the ocean floor. These are not the realms of the famous “black smokers”, but those of a geological cousin, the undersea “alkaline vents” whose existence as the possible environment for the start of life had been predicted but had not been found until deep submersibles discovered them only a few years ago. (Lane has got his hands on the most recent research to write up these chapters, and acknowledges that the explanations are the best science can offer now and will always require modification.
Darwin didn’t know of the vents, we know of them, and maybe scientists a century on will know of some completely different origin site. “I can’t pretend,” Lane writes in his chapter on DNA, “that all of the evidence we’ve discussed here is conclusive, or much more than clues to the deepest past. Yet they are genuine clues, which will need to be explained by whatever theory turns out to be true.”) The vents bubble a supply of hydrogen which could react with the carbon dioxide in the water to form organic molecules, and a cross section of the vents shows a labyrinth of compartments that could have concentrated the organic molecules to become precursors of RNA, the primordial relative of DNA.
After examining the origins of DNA, Lane tackles photosynthesis, the process by which sunlight powers reactions to strip electrons from water and install them into carbon dioxide, producing oxygen and energy-rich sugars. The chloroplast, the organelle in which photosynthesis takes place, evolved only one time, and from that start, now every green plant and alga has them. The bacteria that originally did the photosynthetic trick came to live in larger host cells, engulfed in them but surviving the digestive processes, and ultimately proving useful to the diner. (The origin of the complex cell from simple precursors by processes like this, the borrowing and stealing of genes and information by one ancient cell upon another is the subject of a subsequent chapter.) Not only do plants enable all life by being the source of edible molecules, they are the source of life in a larger sense. The atmosphere before they went to work was a combination of carbon dioxide and nitrogen; once they started pumping oxygen into the atmosphere, not only could microbes start using it for non-photosynthetic energy processes, but they were also protected by the initiation of the shield of the ozone layer.
Why do we have sex? You may form your own ribald answer to the question, but it has been a real puzzle to biologists. After all, if a successful creature could just clone itself asexually, the success could not but continue. There are costs to sex, from the effort expended to finding a mate to the transmission of venereal diseases. But clones, like the dandelion, are relatively new species and they aren’t destined to last long (in evolutionary tempo). Darwin thought sex provided vigor because offspring of unrelated parents would be healthier. He didn’t know about genes, though, and a larger answer is that sex ensures variation in progeny, and evolution operates on variation. More up-to-date thinking has shown that sex protects against parasites, which evolve quickly and would take over populations that don’t have the variation that sex provides. There are plenty of other explanations, and computer-driven mathematics behind them, about which Lane says, “... although it’s a messy solution from a mathematical point of view, nature can be as messy as she likes.”
Lane goes on to discuss movement; initial life forms didn’t power themselves, but the ability to do so was a huge advantage. The contraction of muscle depends on an intricate and coordinated pull of molecules like actin, and the actin in your muscles is almost exactly the same as the actin within immobile yeast cells; it forms the cytoskeleton in such cells, the foundation for movement of intracellular stuff, and evolution has borrowed it to make muscular movement. The closest Lane comes to concentrating on morphology rather than biochemistry is his chapter on vision, but even here he tells about the visual pigment rhodopsin, which evolved just once long ago and is the photoreceptor common to every creature that sees. There are also lens proteins; evolution would predict that such proteins would originally be used in other processes of the body, and such predictions are verified. Hot-bloodedness, or simply the ability metabolically to provide a stable temperature, is the next invention on the list. Homeothermic creatures like ourselves are driven by hunger; we have to power our own warm insides to keep everything perking. This gives us an advantage when weather changes, and it gives us stamina, and also allows big brains. Big brains in humans (let’s not quibble about other animals) produce the next subject, consciousness. We have a single integrated perception from all sorts of input; take a look around you, for instance, and it seems as if you are looking at one scene, but there are two different scenes, one from each eye. This stereo vision is integration is of the simplest form; the higher issue of how feelings, which are just neurons firing, feel so real is full of paradoxes. And finally Lane winds up with, well, finality: death. Simple cells split and multiply, but all the complicated creatures die. Why on earth would we have genes that program senescence and death?
There are wonderful explanations here, and even better questions. In each chapter, Lane has shown some of the history of scientific effort to come to understanding. “... science has a unique power to settle scores thro
Rob Hardy is a local psychiatrist who reviews books for a hobby. His e-mail address is [email protected]
Steven commented at 8/27/2009 2:21:00 PM:
Wonderful insight !