ecognize that fact and build it into our understanding of how the world works.Charles Darwin (left, with son William) knew that his ideas about evolution had a lot to say about humans. But in 1859 in his Origin of Species, Darwin famously only hinted in one phrase that his ideas about evolution would throw light on the origins of humans, too. Twelve years later, that changed dramatically.
In 1871, Darwin addressed human evolution squarely and at length in Descent of Man. There, he compared human morphological traits to those of evolutionarily related primates, he discussed human racial variation, and evolution since the origin of our species. Although he had a sophisticated understanding of the amount of racial variation, he was less sophisticated when it came to culture, and he argued that some races (those that are 'civilized' today) were superior to others ('savages') because of natural selection.
But Darwin used morphology and other human attributes to argue that while we varied a lot around the world, the overall evidence showed that we had a single origin as a species. Races were not separately created.
In that book, Darwin considered behavior when he introduced his theory of sexual selection in the context of human evolution. The idea is that traits can be useful for securing mates, not just evading predators and surviving. This could explain traits, like fancy display traits or even variation in traits like skin color, that seemed to have little actual survival value. Fertility variation as well as mortality variation could be a mechanism of evolution and adaptation.
After Descent of Man was published, Darwin continued to think about behavior and human origins. If Facebook is today's web best-seller, Darwin's 1872 The Expression of the Emotions in Man and Animals was a best-seller of the more traditional kind. This was a detailed attempt to gather locally available data, as well as anecdotal data from around the world, to describe how people use their facial muscles to express their state of mind--their emotions--and thus to communicate visually with each other.
Darwin's motive was in part to show that an 1806 book on the use of facial muscles for expression was wrong. Charles Bell had argued that God had endowed people with expressive facial muscles to show their emotions. Darwin wanted to support his very different scientific view, but more than that, he wanted to continue his argument that humans had a single origin, and one that can be explained strictly in terms of historical processes. While he was an advocate of natural selection, this was almost entirely implicit in Expression, again because the point was to show that we shared these traits worldwide, among all races, and they were similar to comparable expressions in other animals. Ergo: all humans had a single, common, evolutionary origin in mammal phylogeny. We are a unified species.
We write because a new paper in Current Biology by Jack et al. (the abstract is here, and the paper described here by the BBC), reports on experimental data that suggests that Darwin's argument was, in fact, at least substantially wrong. In a controlled study of recognition of standardized facial expressions, the authors found that East Asians and Europeans tend to look at different parts of
the face (eyes, mouth) for indicators, and interpret some expressions differently. The figure from their paper shows how these differences are statistical statements of how often or likely members of each group focus on, say, eyes. But it shows that these things are not hard-wired as Darwin had attempted to show in his way and with data available to him.
The shared-trait approach is called cladistic by taxonomists trying to reconstruct evolution. The idea is that shared traits mean shared ancestry. But just because organisms share some trait doesn't mean they have common ancestry because there can be independent evolution of the trait. This is known as convergent evolution or homoplasy. But even shared traits and common ancestry don't imply single species origin. Dogs and cats have legs and sharp teeth, but are not a single species--even if their shared traits reveal deeper common ancestry.
Of course, all life shares common ancestry if current evolutionary theory is at all correct, as it seems surely to be. So the question has to do with species' identity. One has to work out the evolution of each trait separately. And environments can play a role, especially in something like facial expressions, as they can be taught rather than inherited.
Nobody should have accepted Darwin's argument wholesale. Expressions can be faked. Actors and con artists do it all the time. We do it to smooth over awkward social situations and in many other times when we weave the tangled web by practicing to deceive. It's common experience not to be sure what someone's glance may or may not mean. Most species with once-thought universal behaviors, like bird songs or cattle sounds, have regional dialects--their version of 'culture.'
But there's also a deeper point. In evolution, few rules are iron-clad. It's not like, say, universal gravitation or chemical bonding between atoms. So the fact that we may not, after all, universally use the same facial expressions doesn't provide any support at all for multiple human origins! We know that we have a single origin, but we know it from tons of evidence of other kinds that is much clearer and more convincing.
The deeper lesson is to avoid the common mistake of over-specifying evolution and its principles, of making scientific generalizations into scientific dogma.
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