Theories about human evolution
In one sense there is single broad theory of human evolution – a Darwinian one – but many competing specific ones. These may relate to different phases of human evolution – for example, a theory for why bipedalism evolved more than four million years ago, and another for why brains became larger less than 2 million years ago. They be competing theories for the same facts – one might propose that bipedalism is an adaptation to foraging on the ground, another to the pressure of high temperatures on body shape, another to wading in marshes. Research in human evolution focuses on acquiring new facts to test these theories, and developing better and better methods to make such observations.
Theories in human evolution can relate to events in the past – looking at the overall sequence of events and looking for ways of explaining the pattern (for example, in terms f climate, or competition with other animals, or the pressure of social life). Increasingly important are theories about how human behaviour today may bear the imprint of evolution, and so living humans can be studied. A classic example of this is work originated by Darwin himself, who showed that facial expressions were universals, and expressed a few key emotions, easily understood by all. Theories to account for this type of behaviour are now strongly Darwinian.