User:TonySidaway/Evolution

Evolution is the common name for the emergence, life, and eventual extinction, of different species on earth over time. We know that species become extinct because this has been observed directly and the proximate cause of a particular extinction is usually evident. We can also observed directly that living organism are not randomly distributed across the Earth but are adapted to their surroundings. But we now also know that new species come into existence over great expanses of time, and that all individual organisms are connected by a chain of ancestry. We also know much about the mechanisms that drive this evolution. This essay outlines the chain of evidence and reasoning that has led us to make this deduction.

The fossil record

Fossils were known in ancient times, and various explanations were offered to explain their existence. Our modern appreciation of how fossils form is based on the key observation that fossils tend to be confined to certain strata of rock. When European explorers started to examine strata outside Europe, they quickly began to see that rock strata are laid down in a predictable order and that the order represents the chronology of Earth's long history. Thus fossils represent the forms of life that existed at the time the strata were laid down. This chronology has come to be known as the fossil record.

Geological time

Early scientific estimates for the age of the Earth differed widely because although the geological strata indicate a chronology, there was no way to assign an age to different strata. Similar problems plagued attempts to measure the age of other astronomical bodies such as the sun and the other stars. In the past century this problem has been solved by the use of multiple radiometric markers. The principle is that radioactive decay follows an inverse exponential relationship, with a predictable half-life. While any one marker may produce a misleading result as a side-effect of contamination, by measuring multiple radioactive markers in the same rock sample we can reduce the uncertainty. Modern mass spectrometry can produce remarkably accurate assays of very small samples. The results obtained for the ages of rock samples are consistent with those from meteorites which indicate that the solar system is some 4 billion years old.

Thus we can now estimate that the rocks in which the earliest fossil microbes are found are about 3.5 billion years old, that the earliest known widespread fossil fauna of solid-bodied animals were laid down in the Cambrian sediments over 500 million years ago, the first bony fish appear some 440 million years ago, and so on. We can also date the appearance, long histories, and eventual extinction, of whole groups of species, most notably the trilobites.

Common descent

Very close similarities between species have always been observed. Specific details of living organisms serve quite different purposes, such as the jaw bones of certain fish, whose analogues in mammals are part of the inner ear. The presumption of a familial relation existed long before plausible mechanisms were understood and confirmed. Organized taxonomies, based on such homologous traits, drew attention to this persuasive though at the time mysterious evidence suggesting common descent.

By the early nineteenth century fossil collecting had spread widely enough to make the recognition of a succession of different forms of life to be a commonplace. Many naturalists noticed that some of the fossils seemed to be extinct members of known species, though often of different size from any of their living relatives, or bearing characteristics unknown in the surviving species.

Population thinking

There were still barriers to thought, hurdles that prevented the understanding of common descent gaining ground. Chief among these was what might be called the platonic species concept. In this view of species, individuals are all members of one and only one species, and the range of variation of any species is quite small. This is expressed in a very strict form by the classical Greek philosopher Plato, but it conforms to everyday experience and most people's instincts.

An alternative to this species thinking which is often adopted by modern biologists, is population thinking. A population is a group of organisms that interbreed. If river flooding or some other force should isolate some members of a population from others, then the members of the isolated group become a new population. Since descendants get their characteristics from their ancestors, this provides a coherent alternative to the platonic species concept. This new thinking allows naturalists to think about what happens to a population that is isolated in such a way? Does it mirror the population from which it is derived, or does it change? Could such change help to explain why organisms, both in modern day and as represented by fossils, appear so well adapted to their circumstances?

Natural selection

Species have a degree of plasticity, this was known from the earliest times, but understanding adaptation--how species tend to be fitted to their circumstances--took many iterations. Somehow, it seemed, successive generations must become better at doing what they did, but how could this be so if each generation had to start anew as a seedling, or as a newly hatched or newly born animal?

A milestone on this route, which was not fully abandoned until the mid-20th century, was the notion that the experiences of the parent are somehow transmitted to the offspring. This led to an explanation of adaptation that relied on the observation that many organisms adapt during their lifetimes. Thus in cold conditions many species of mammal do react by growing more fur.