You'll have often seen bits of the phylogenetic tree, showing how humans are related to other primates:
( gibbon - orangutan - human - chimp - bonobo - gorilla )
But how do we know? How can we say for certain that chimps split off from bonobos more recently than humans split off from gorillas?
Why do we think humans are more closely related to orangutans than to gibbons? Is it just that we look more similar to orangutans? Could it be just that the Intelligent Designer created some species to be more similar to other species, and that any similarities in their DNA are just because the Intelligent Designer re-used some of His blueprints?
What's the evidence that these are related by one creature
giving birth to another creature, from parent to child to grand-child,
through hundreds of thousands of generations, slowly forking within
seperated sub-populations until multiple distinct species emerge?
The answer is disease.
Back in 1999 two scientists, Welkin Johnson and John Coffin, took syringes of blood from humans, chimpanzees, gorillas, orangutans, bonobos, gibbons, macaques, baboons and marmosets. They took the resulting DNA and searched it for markers left in it by a particular sort of disease (an ERV) which alters the DNA of an infected creature by inserting short sequences into it. They then split their results up by chromosome region and, for each region, fed the results into a piece of software that generated a phylogenetic tree based solely on inheritance pattern of markers found in that one region.
So, for example, if in a particular region the software was told that:
Then it deduce that markers D and J were introduced after Gorillas split off from the other three, and that marker B means Chimps and Bonobos share an ancestor that that Humans don't.
But my example doesn't give an idea of the scale. The genome is about 2,500,000 base pairs long and there are over 3000 of these markers in the genome, that we've discovered so far. So it isn't just 1 or 2 markers that make the difference in the order they split off. It is hundreds of markers, each of which is in exactly the same position in the two species, out of 10s of 1000s of possible combinations.
That's like someone living in a shared
flat, and the two flatmates mark all the property in the flat with the
flat's zip code in invisible marker. Even years later after they have
moved to different cities you'll still be able to tell which two (of out
ten otherwise randomly selected people) shared a flat, because they
each have 20 or so CDs in their music collections with the same zip code
marker on it. That just doesn't happen by coincidence.
So what did Johnson and Coffin find, when they constructed a tree based on each region? Were the trees different, as you'd expect if this was just as coincidence from where disease happened to strike in different species? Or were the trees compatible, as can only be explained if the species were actually descended from common ancestors which a million years ago got infected by a disease that marked their DNA, passing on the same marker to each of the ancestor's children?
can see, the resulting generated phylogenetic trees are all compatible.
None of them show bonobos being more closely related to gorillas than
they are to humans, for example.
Here's the full scientific paper, published in a reputable peer reviewed journal of the relevant scientific area, that has since been independently replicated: