The Real Missing Link

A Therapsid from the early Triassic -- Picture courtesy of Wikipedia

by blogSpotter
In discussions of evolution, the "Missing Link" always refers to some ape-man that provides a mid-point from ape to human. This conjecture is hardly interesting to me, though it's interesting and controversial to people who don't believe in evolution. Ape DNA is 98% the same as humans, apes originate from the same geographic areas as early humans, and most of all they physically resemble humans. Apes have the same gestation as humans, 9 months, and they even can be trained to speak primitive language. My imagination doesn't have to take any gigantic leaps to see a chimpanzee-like animal turning into a human.

Now, there is a missing link in nature that I find pretty puzzling -- it has hardly been addressed. I call it the reptile-to-mammal connection. Between fish and salamander, we have lungfish. Between tree mouse and bat, we have a flying squirrel. We have almost continual evolutionary calling cards throughout much of the animal kingdom. But for some reason, between tree lizard (an advanced reptile), and opossum (a primitive marsupial) we have nothing. For this discussion, I'm regarding marsupials as primitive mammals. No living animal species represents a plausible bridge of these two. The Australian platypus lays eggs and has venom glands, but still is overwhelmingly mammal in its behavior and looks.

The systemic changes that took place toward evolution of mammals were monumental: 4-chamber heart, mammary glands, external ears, internal temperature regulation, fur, etc. It's interesting to conjecture how it came about. One has to figure -- at no point did a marsupial crawl out of an egg laid by a lizard. There was a geneticist, Richard Goldschmidt, who once made such a conjecture -- the "hopeful Monster" theory. It has never been observed in nature, and it was mostly dismissed by other scientists.

There is ample fossil evidence of mammal-like reptiles that existed early in the age of dinosaurs. They were called synapsids and therapsids. They truly looked half lizard, half mammal -- their legs were notably more elongated than a lizard's, under the trunk of the body like a mammal. They must have been somewhat successful because they diversified and their fossils are everywhere to be found. None of these species survived into modern times though some lasted into the Jurassic era. My own conjecture is that the full progression from therapsid to mammal happened very rapidly and in one specific locale. ("Rapid" in genetic terms might be a few thousand years). So rapid and restricted was the change, that the "promotion" to advanced features was wholly inclusive to a tiny population, and all animals in that population were "promoted". If it had happened over a longer time frame, there would have been population drift and surviving "cousin" species.

What a shame that we have no living clues, only fossils. Arguably the largest, coordinated macro-mutation accomplished by evolutionary process, and we have nothing but two end points to look at. That fact, along with the complete disappearance of dinosaurs (arguably another transitional advancement over reptiles) suggests some very interesting biological events took place throughout the pre-Cenozoic era. The therapsid and its immediate successors are the real missing links – not the ape man. Maybe someday we can reconstruct what happened in that pivotal moment of evolution.

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