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?)
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.
Question from EvoE: