Evolutionary thought has been around since before the time of Jesus. In the 6th century BCE, Greek philosopher Anaximander speculated about the origin of life and he believed that animals originated from the sea. A succession of Greek, Roman, Arab, and Persian philosophers put forward their ideas on evolution. Such ideas became more sophisticated in the 18th century as Pierre Maupertuis and Erasmus Darwin took advantage of the greater knowledge of biology by this time. Biologist Jean-Baptiste Lamarck wrote about the transmutation of species in 1809.
But it was Charles Darwin's ground-breaking book, On the Origin of Species, in 1859 that shook the establishment to its core. He proposed that life evolved from common ancestors, including humans. He coined the term natural selection to describe how animals passed on their traits from one generation to the next. Evolution soon became accepted by scientists and the general public. Today, evolution is accepted by at least 95 per cent of biological and earth scientists, with one survey suggesting the figure is 99.8 per cent. However, a resurgence in creationist beliefs since about the 1920s has led to at least 40 per cent of people in the United States supporting creation, although the figure is generally thought to be lower in other countries.
Evolution refers to the changes in a population's traits or characteristics between generations. These changes can be caused by genetic influences or by environmental factors or both. Inherited traits in any individual come from the parents' genes, which are passed onto it automatically. Mutations, or changes, in genes can occur due to things like chemical agents, viruses, or radiation, and can result in altered traits in offspring. Migration is likely to speed this process up as genes from different groups are passed onto the next generation. Some mutations will decline in a population while other more favorable ones will increase by natural selection and may lead to evolutionary change. Favorable mutations are those that help a species to survive in its environment and to reproduce.
The other main factor in the evolutionary process is genetic drift. Under this principle, introduced in the 1920s by American geneticist Sewall Wright, random chance determines which gene variants will be passed from the current generation to its offspring. To see how this works, imagine a barrel with 10 blue balls and 10 brown balls. Look away and take a ball from the barrel. Put a ball of the same color in a second barrel and return the ball you took from the first barrel to that barrel. Shake the barrel and pick a ball. Do this 20 times and see what combination you end up with in the second barrel. It could be 10 and 10, or 11 and 9, or some other combination, although the chances of a particular combination decrease the further it is from 10 and 10. The second barrel will be the new generation. Repeat the first step and another different combination of balls will likely result. Keep repeating the process. Sometimes there will be more blue balls, and other times more brown balls. It's possible that one of the colors will disappear altogether after a large number of generations. This is how genetic drift works.
Natural selection and genetic drift occur at the same time in any population. In a small population, genetic drift will dominate. However, in a larger population, natural selection will tend to overshadow drift, even when selection is weak. Try the above experiment with 50 blue and 50 brown balls and you will see that the relative effect of drift will be less than what it was with 10 blue and 10 brown balls.
How did life come from non-living matter in the first place nearly four billion years ago? I wish I knew. Whether scientists will eventually come up with a satisfactory explanation is hard to tell. It could have been the result of some sort of spontaneous chemical reaction or self-replicating molecules (e.g. ribonucleic acid, or RNA) or self-assembly of simple cells. Lots of things are possible in an open system. But understanding how evolution occurs, and the fact that it does occur, doesn't depend on knowing how life started.
All organisms have a common ancestor or gene pool. The first organisms on earth go back 3-4 billion years. These were the prokaryotes, single cell bacteria and archaea that can live in inhospitable environments. The next step was eukaryotic cells which evolved from ancient bacteria. Various multi-cellular organisms developed independently in the oceans from around one billion years ago. Evolution accelerated during a 10 million year period known as the Cambrian explosion about 530 million years ago. Complex forms of animals developed at this time. Some 500 million years ago, plants appeared on the land. Animals such as certain arthropods soon followed. Other animals appeared later, such as amniotes from 340 million years ago, amphibians 300 million years ago, mammals 200 million years ago, and birds 100 million years ago. The evolutionary process is ongoing. What we see today is a collection of species at their current stage in the process. New species will form and others will become extinct.
Perhaps the best way to see what evolution is and how it occurs is by way of example. Let's assume a particular species lives in a certain area and goes about its daily routine of survival: eating, sleeping, and reproducing. If food becomes short due to over-populating or drought or some other reason, some of this group will have to move and find another home if all members are to survive. Let's say a few hundred of the species move on and a few hundred stay put. The migratory group finds a new home that is a bit warmer and wetter, has more food, has fewer natural predators, and has trees that are easier to climb as the timber is softer. The environment of the sedentary group is somewhat the opposite, and actually continues to become drier. For the purposes of the example, the new environment of the migratory group might be dry and the old environment wet. It doesn't matter.
What would happen to the two groups? Assuming they are a reasonably hardy, adaptable species and would survive, the two groups would gradually adapt more and more to their respective environments. Over time, the migratory group would probably breed faster, eat more, perhaps get larger and fatter, and maybe slower and lazier. Their claws might lose strength and sharpness over time. The sedentary group might not breed as fast as there is less food and water. They have to watch their backs more and will become quicker and perhaps develop better eyesight or smell or both to avoid predators and to catch their own food. And they would develop sharper claws, and stronger limbs, to climb the hardwood trees. Eventually, physiological changes in the two groups might make them sufficiently different that a male and a female from each group could no longer breed. The result is two species from one. This might take many tens of thousands, hundreds of thousands, or even millions of years, depending on the circumstances.
But food is running out in the habitat of the sedentary group, so some of them migrate in search of a better food supply. They find it, and live in this new, different environment for a long period and adapt to it. Eventually, they are sufficiently different from the rest of the group they left behind, that the two parts of the group become separate species. The part of the group that migrated is then hit by an ice age and a large number of them set off to find a warmer climate and more food. And so the process continues.
Some groups and part groups continue to adapt to their ever-changing environment, while others die out. Some subgroups have to become smarter to survive. Constant use of their brains results in them finding new ways to survive. They use sticks and rocks to help them kill prey, and fire to cook it and to keep warm during cold winters. These particular subgroups no longer live in trees as there are predators waiting for them, so they start living in caves instead where they can throw sticks and stones to ward off these predators, and put up barricades at night, knowing that other, bigger, stronger species don't have the know-how to tear them down.
One particular subgroup develops its brainpower at a faster rate than similar species and secures the lion's share of the available food. The several species that make up the close cousins of this superior species become increasingly hungry. Their numbers dwindle and they eventually become extinct. The surviving species is modern humans. Their close cousins, the Neanderthals and others, have disappeared. Meanwhile, slightly more distant cousins within the Hominidae family are doing their own thing in their own environment, perhaps including members of the initial species described in this example, and are still climbing trees and have plenty of food and man is not a threat (until far more recently). Evolution has taken place!
We might be seeing the potential for further evolutionary change in humans in the last hundred years or so. We saw an increase in height and weight as we improved our nutrition, general living conditions, and health. More recently, we've seen further considerable increases in weight due to junk food and sedentary lifestyle. If you divided a group of humans into two further groups and put up a wall between them, and let one group continue a lazy, junk food existence and let the other group become fitness fanatics, changes in each group might become sufficient that breeding between the "lazy" and "fit" groups may no longer be possible in perhaps as little as several tens of thousands of years, assuming the "lazy" group survives its excesses.
The evolutionary process is sometimes divided into microevolution, which describes small changes over a short period of a few generations, and macroevolution, which refers to the larger changes that occur over a longer period. The creationist model supports microevolution but not macroevolution where new species might be formed. However, the two terms describe the same process. Any division is arbitrary and, according to the American Association for the Advancement of Science, has no scientific basis. In the example above, two groups of the same species aren't going to stop evolving simply because they've reached the boundary of their ability to reproduce together. They may continue to evolve and may end up as two separate species.
Evidence of evolution is abundant. You may have seen those Christian websites that quote from works by scientists admitting to a lack of transitional forms. The sites then claim that the absence of these forms means that evolution is nonsense and therefore everything had to be created by a creator. But note that the references are always old, usually from the 1960s through to about 1980. Research into transitional forms is expensive and has long lead times. Since pioneering research in the mid and late 1970s, numerous transitional forms have been identified. Sure there are many missing links, and probably always will be. But knowledge of evolution has come a long way since Darwin first made his observations on fossils and species, and saw evidence of evolution, rather than accepting without question what was stated in literature from 2,000 years ago.