The famous author of science and science fiction, Isaac Asimov, had a fondness for some of our remote ancestors, even pleased that they might still be with us. In an essay called “Holes in the Head” back in September, 1971, he verged into the nature and evolution of funny mammals called monotremes. These are the well-know duck-billed platypus and the spiny echidnas. They’re mammals, as they nourish their young with milk, even if the young develop from eggs like reptiles do.
Nature doesn’t make nice sharp distinctions as we might wish. All of us, humans, bears, kangaroo rats, monotremes arose from a line of reptilian-looking therapsid animals that started 280 million years ago. All the branches of this related group or clade disappeared by around 100 million years ago, except we mammals. Our earlier ancestor therapsids are gone… or are they?
Asimov cites studies by the zoologist Giles MacIntyre, who studied the structure of the skull in ancient therapsids, the duck-billed platypus, and “regular” mammals. He followed the route of the very important trigeminal nerve between the brain and the jaw. In the immature platypus the nerve goes between bony plates. In the rest of us the nerve goes through a hole in our head, hence the title of the essay.
Asimov likes the idea that the platypus is an early therapsid, still with us. It’s akin, in a way, to our having part Neanderthal in us humans – 1-2% in the genes of us Europeans and Asians.
This has been an outreach activity of the Las Cruces Academy.
