Let's look at a thumbnail sketch of the slow and steady march of evolution and natural selection on the Earth, from its humble origins nearly four billion years ago, to the complexity of life on Earth today. Our theory of evolution is Darwin's theory. Dating from 1859, The Origin of Species, one of the most marvelous books written in science history. A small book, Darwin was sitting on the results for nearly 20 years when a rival, Wallace, was about to publish. Forcing him to condense his arguments into a small volume, 150 pages long, full of brilliant observations and cogent arguments. We can watch a condensed version of the evolution of life on Earth from the simplest microbes, perhaps near hydrothermal events emerging on the earliest oceans, to the gradual evolution of life onto the land. Which happened only 400 million years ago, and eventually to the formation of complex and intelligent creatures like us. Remember that for most of the history of life on Earth, no organism was larger than a microbe. There are several important landmarks in the history of life on Earth. Evolution is not a slow steady progress continually increasing in complexity and size. There have been long periods of apparent stasis, and times when life is under duress such as just after a mass extinction, which has happened at least five times in the history of the Earth. But the major innovations in life as it evolved, involve going from cells without nuclei to cells with nuclei, which concentrated in a small chemical factory, the functions of a cell, and proliferated the number of types of cells. Humans for example have over 50 types of cells. A second innovation was organisms that were able to metabolize gas and release oxygen. That's where the oxygen we breathe came from, and of course this oxygen was poisonous to some of the organisms that existed at the time. The very first microorganisms probably metabolized methane, which was present in abundance on the early Earth. If the history of life on Earth is presented as a linear timeline, what's striking is how from most of the history life was fairly simple and microbial. The presence of large creatures on the land is a relatively recent occurrence, and the presence of intelligence and sentience is an amazingly recent occurrence in geological time. Another remarkable innovation dating back about 1.5 billion years is photosynthesis. This is an extraordinarily efficient mechanism for creating biological material. In it, carbon is essentially snatched from the air. It's taken from carbon dioxide, and then the object respirates oxygen. If we follow the process, we can see that carbon snatched from the air is being built into cellulose and other organic material that form the substance of a plant. The equation of photosynthesis shows how this occurs. This is extraordinarily efficient because it directly harnesses radiation and turns it into biological material or organic material. The way of framing this question so you can see where it comes from, is to imagine that when a tree grows from a tiny seed, where does most of that mass come from? Does it come from water? Does it come from the air, the Earth or was it already present in the seed? Astonishingly, it comes from the air. Photosynthesis is now the base of the food web on Earth. The evolution of life on Earth does not take place in a sealed environment. Earth is subject to influences from space, and cosmic intruders can severely disrupt the process of evolution. If we follow the evolution of species since life's moved onto the Earth, or since creatures have been larger than microscopic the last half billion years, we can find at least five times when more than a quarter of the species were extinguished in a very short geological time. Including one episode where over 90 percent of the species were extinguished. These mass extinctions do not all have explanations. We're fairly confident that the KT extinction 65 million years ago was caused by an impact. The crater and debris and radioactive dating all point to this, just off the Yucatan Peninsula in Mexico. But the other mass extinctions don't have a similar cause, although they may be impacts. At least one, the Permian extinction, is likely to have been a dramatic volcanic episode that teraformed half of the northern hemisphere in a short period of time. Life survives during these mass extinctions. Evolutionary biologists have argued that the duress of a mass extinction creates new opportunities for species that survive. They essentially can radiate into new ecological niches. So assuming that it's just a bottleneck and not total extinction, the result is increasing diversity following the mass extinction, and we see this in the fossil record. The astronomical part of this scenario can be pieced together because we know how much debris there is in space left over from the formation of the planets. Most of this debris is in the asteroid belt, and these objects occasionally come on Earth crossing trajectories. The other main repository is the comet cloud, consisting of perhaps a trillion objects 5-10 kilometers across, that come on looping elliptical orbits occasionally into the inner solar system. The impact rate of large objects that could destroy a continent or perhaps an entire biosphere is fairly well known statistically. A catastrophic impact sufficient to cause a mass extinction should occur on average about once every 100 million years. Early in the history of the Earth when there was more debris, that may have happened 10 times more frequently. In fact, it's predicted that we were sterilizing impacts that occurred on the Earth in its first half billion years. Perhaps eradicating life completely, and meaning that the current version of life was a second or a third start. Although the Earth is a restless planet that removes the evidence of impacts, there are remains of impacts such that we can piece together the impact rate. Including smaller objects that cause devastation on local scales the size of a country. The poster child for mass extinctions and their explanation in astronomical terms is the KT impact boundary at 65 million years ago. There, several decades ago, the Alvarez father and son, geologist team found an enormous concentration of iridium at the boundary layer, 65 million years ago. Iridium is a rare element on the Earth. But it's more abundant in extraterrestrial materials such as meteors. This was an indication of an extraterrestrial intruder. Also at that boundary layer they found soot. Evidence of worldwide fires because the same soot was found at different sites around the world. Further, there was evidence of shocked quartz. The kind of quartz that could appear from volcanism, but in widely dispersed sites must be sign of larger energy deposition on the Earth. These pieces of evidence taken together, all occurring within a short geological interval, indicated that an impact caused the devastation that extinguished the dinosaurs and about a third of the large species on Earth. We can imagine with what we know about exoplanets and other solar systems, that these planets will also have impact environments. We imagine that if life developed on other planets, it will also be subject to influences from space. Some disruptive, some catastrophic. Stephen Jay Gould famously talked about the contingency of evolution of life on Earth. He imagined that if 100 Earths had been placed there around 100 identical warming suns four and a half billion years ago. He didn't imagine that you might have mammals on more than half of them, and certainly you wouldn't have humans on more than a couple. He thought that there were so many branching points in evolution and so many uncertainties, that it was hard to predict what the outcome would be given the initial conditions. Other biologists like Simon Conway Morris, take the opposite view and argue that evolution is convergent. As we can see, with only one example of biology to study, we may never be able to decide between these quite differing views about how evolution works. However, the map of how species evolved is well done through DNA, and we can see all the branching points that led to the first creatures that were larger than microscopic, which only happened about a billion years ago. The first creatures that moved onto the land and had skeletons about half a billion years ago, and the first creatures that in any way resemble us, precursor mammals about 50-70 million years ago. The final stages of evolution leading to us looking backwards now, involve the gradual deviation of ape species from a tree that eventually leads to primates, chimpanzees, and us. The last divergence in this tree of life leading to us occurred about a million years ago. There have been several great innovations in the evolution of life on Earth over 4 billion years. For most of that history life was microbial. The first great innovation was the development of cells with nuclei, allowing much more sophisticated cellular function. Another innovation was the development of photosynthesis. Whereby the sun's radiation could be trapped directly to create organic material. The development of creatures that can move on the land and of plants and animals is a fairly recent occurrence in the last 10 percent of the history of life on Earth. Life on Earth does not proceed uninterrupted. The cosmic environment plays a role, and there's evidence of at least five mass extinctions of life on Earth. Many of which were probably caused by impacts from asteroids or comets. Because of this, life's evolution is unlikely to be a predictable outcome given initial conditions. It's likely to have many contingencies and divergences and possible paths. By implication, life on other planets will operate similarly. But we don't know.
