Science and Human Evolution

Question from EvoE:
What is the proof humans have evolved from a common ancestor? If the scientific method is observation, data collection, internal and external validity and reliability; then how can one observe human evolution?

Answer by SmartLX:
We don’t observe human evolution. Scientists genuinely don’t know whether it’s still happening, and discuss it often.

This means little, because science does not always require direct observation of a past event to confirm that it’s happened. If it did, the police would have to be present at every murder in order to charge any suspects. We can instead observe and collect evidence that humans have evolved from ancestors we share with other animals.

The evidence includes but is not limited to:

1. Comparative physiology
We often say we evolved from apes, but it’s just as correct to say that we are apes. Without even considering genetics, we can be classified as apes using physical characteristics such as our long arms, omnivorous teeth and lack of a large tail. It goes far beyond superficial physical qualities, because our internal organs and body chemistry have countless analogues in other primate bodies. We’ve even got some organs that are practically useless to us but essential to other primates, such as the appendix and the muscles for moving our ears. (These and other human vestigial organs speak loudly against the Intelligent Design proponents’ claim that all the similarities are due to the other animals having the same Designer. Why wouldn’t He just leave out the useless organs, rather than leave them in place to potentially kill us?)

2. Genetics
The genes linked to the analogous systems mentioned above can be sampled from each species and compared directly. Now that the whole human genome has been sequenced, it can be compared all at once to the genomes of other animals. The results are as expected: our DNA diverges less than 2% from some species of apes, such as chimpanzees and gorillas. (By comparison, the most distantly related humans differ from each other by about 0.1%.) As we compare ourselves to other non-primate mammals, reptiles, plants and so forth, the genetic difference rises accordingly, exactly as we would expect if we diverged from these other branches of life earlier on.

Even knowing single comparative facts, like that chimpanzees have one more chromosome than we do, we can predict and discover evidence of very specific biochemical events that happened during our own evolution, such as the mark where two of our ancestors’ chromosome pairs fused to make one of our own pairs, specifically Chromosome 2.

3. Observed natural and artificial selection (and speciation)
We have watched some species with relatively short life cycles breed and diversify to the point where two populations of the same animal became unable to breed with each other. We’ve conducted breeding programs which have deliberately achieved the same thing, just to see what’s needed for it to happen. These are instances of speciation, or the splitting of one species into two, because part of what defines species is that they can’t produce fertile offspring with other species. There have been billions of years for this kind of thing to happen all over the world without any external help, and there’s no reason to think our own origins are any different.

If you want the rest of the evidence, start at the Wikipedia page for evidence of common descent. It’s not solely concerned with humans, but a lot of it has to do with us. If there’s something specific there that you don’t accept, bring it up in a comment.

3 thoughts on “Science and Human Evolution”

  1. “scientists are not sure if evolution is still taking place”??? Really? Could you provide an example of one these discussions that you say take place between scientists regarding this issue? Thanks.

  2. Sure, here’s an article on one such debate. Professor Steve Jones, mentioned within, appears to be the best-known proponent of the view that human evolution has essentially stopped.

    It’s important to insert the word “essentially” because no one disputes that it still happens in minor ways, in the sense of being unlikely to affect the appearance of the human race as a whole in the long term.
    – Mutations still occur and some are very obvious, like extra fingers and weird eyes – though ones like these seldom benefit people in terms of natural selection. (One old man was on TV talking about his six-fingered hands and how, as a boy, no one dared make fun of him as he could make the biggest fists. In a different kind of society, he would have had a distinct advantage.)
    – Resistances to diseases are hereditary and can change between generations, often very quickly if it means the difference between survival and death. (There are already some Africans with great resistance to HIV, which allows a few to work as prostitutes far more safely. Sadly it has not reached the point of immunity, as shown here.)

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