I see, with rather alarming frequency, a major fallacy creeping in to discussions of human evolutionary history and how one may infer details about it. Specifically, there is a tendency to examine the traits of one or a few non-human species and to draw conclusions about the origin of human traits purely from these observations. Recent examples include observing some chimps carrying nuts and drawing conclusions about human bipedalism, or watching monkeys flap their lips and linking this to the evolution of human speech.
The fallacy here is to assume that the non-human species is “primitive”, such that it can be used as a proxy for a distant human ancestor. This is a fallacy because “primitive” and “derived” refer to individual traits, not entire species, and because the comparison being made is between two modern species, not an ancestor and a descendant. Both lineages have, by definition, been evolving for exactly the same amount of time since diverging from a common ancestor.
I am going to call this the “Platypus Fallacy”, based on an example that I have used in my lectures. Consider the following quote, by one of the authors of the platypus genome sequence paper:
“The platypus is a very ancient offshoot of the mammal tree, so it was 166 million years ago that we last shared a common ancestor with platypuses, and that puts them somewhere between mammals and reptiles, because they still maintain quite a lot of reptilian characteristics that we’ve lost; for instance, they still lay eggs. So we can use them to trace the changes that have occurred as we went from being a reptile, to having fur to making milk to having live-born young.”
Now, if a platypus were asked to summarize the situation, he could just as well say this:
“The lineage of which humans are a part is a very ancient offshoot of our mammalian family tree, so it was 166 million years ago that we last shared a common ancestor with humans, and that puts them somewhere between mammals and reptiles, because they lack a lot of specialized characters that we have gained but the ancestral amniote also lacked; for instance, they have no electroreception, no bills, no webbed feet, and no venom. So we can use them to trace the changes that have occurred as we went from being a reptile, to having fur to making milk to having our specialized features.”
Of course, if a platypus did say such a thing, I hope his colleagues would encourage him to avoid committing the “Human Fallacy”.