With the publication of "On the Origin of Species" by Means of Natural Selection, Charles Darwin in 1859 showed conclusively that species evolved and were not immutable over time.【F1】
This revolutionary idea permitted an explanation of the fossil record that did not need to invoke the Biblical story of the Flood or the view that all extinct animals and plants had perished as a result of this one global catastrophe.
It became possible to compare modern and fossil animals and to construct lineages through time that documented the changes that had occurred, and the distribution of fossil forms began to take on new significance. It became apparent that assemblages of fossils betray climatic preferences at any given time and climatic change through time.【F2】
Another 12 years elapsed before Darwin applied his theory of evolution—and its mechanism, natural selection acting upon a pool of normal biological variation—to the case of man; the delay more likely was because of lack of fossil evidence than lack of courage.
【F3】
The so-called Darwinian tautology, "The survival of the fittest is the survival of those best fitted to survive", gives an insight into the adaptations of living organisms that lead to an increase in their chances of survival and of leaving more offspring than their rivals.
The closer the adaptation to the environment, the greater the chances of survival. This pathway leads to specialization: fish need water in which to swim, birds need wings with which to fly, koalas need eucalyptus leaves to eat—nothing else will do.
This approach to survival has its advantages but also its drawbacks.【F4】
Should the environment change suddenly, those who have gambled on specialization may lose, while those who have retained a generalized form and remained adaptable can adjust to the new situation and survive.
【F5】
On the whole, the order of Primates, which contains humans and their ancestors, has retained this approach, an evolutionary flexibility that has enabled primates to respond to change when it has arisen.