Combined with Darwin's original insights, genetic advances led to what has variously been called the modern evolutionary synthesis or the neo-Darwinism of the 20th century. In these terms evolutionary adaptation may occur most obviously through the natural selection of particular alleles. Such alleles may be long established, or they may be new mutations. Selection also might arise from more complex epigenetic or other chromosomal changes, but the fundamental requirement is that any adaptive effect must be heritable. The concept of devolution on the other hand, requires that there be a preferred hierarchy of structure and function, and that evolution must mean "progress" to "more advanced" organisms. For example, it could be said that "feet are better than hooves" or "lungs are better than gills", so their development is "evolutionary" whereas change to an inferior or "less advanced" structure would be called "devolution". In reality an evolutionary biologist defines all heritable changes to relative frequencies of the genes or indeed to epigenetic states in the gene pool as evolution. All gene pool changes that lead to increased fitness in terms of appropriate aspects of reproduction are seen as (neo-)Darwinian adaptation because, for the organisms possessing the changed structures, each is a useful adaptation to their circumstances. For example, hooves have advantages for running quickly on plains, which benefits horses, and feet offer advantages in climbing trees, which some ancestors of humans did. The concept of devolution as regress from progress relates to the ancient ideas that either life came into being through special creation or that humans are the ultimate product or goal of evolution. The latter belief is related to anthropocentrism, the idea that human existence is the point of all universal existence. Such thinking can lead on to the idea that species evolve because they "need to" in order to adapt to environmental changes. Biologists refer to this misconception as teleology, the idea of intrinsic finality that things are "supposed" to be and behave a certain way, and naturally tend to act that way to pursue their own good. From a biological viewpoint, in contrast, if species evolve it is not a reaction to necessity, but rather that the population contains variations with traits that favour their natural selection. This view is supported by the fossil record which demonstrates that roughly ninety-nine percent of all species that ever lived are now extinct. People thinking in terms of devolution commonly assume that progress is shown by increasing complexity, but biologists studying the evolution of complexity find evidence of many examples of decreasing complexity in the record of evolution.