biology azoology blog organs teeth growth allometry

Give a Little – and Take a Little

Increases of some structures appear to parallel decreases in others

It seems odd, but no animal with horns or antlers, whether fossil or recent, has all the teeth required to give it the complete and full dentition characteristics of mammals. No species of spider is known to possess the ability to sting, but all have a poisonous bite (although, luckily only a few possess poisons potent enough to harm humans). The most colourful birds like tropical parrots and Papua New Guinea birds-of-paradise leave a lot to be desired when it comes to their ability to sing and the increase in swim speeds of fish like the tuna apparently went hand in hand with the loss of the buoyancy organ, the swim bladder. There are lots more of such examples, but what am I trying to demonstrate with these examples? I’m trying to show that the development in one area frequently has consequences in another and that a “push” in one evolutionary direction can see a reversal in another.

What more than anything else sets the human being apart from all other animals is firstly the human’s enormous hypertrophy of the brain, in particular, the “neocortex”, in other words, our “thinking cap”. Secondly, we have an unspecialized, almost primitive hand, for virtually all other mammalian species have specialized hands and/or feet, be it for swimming, digging, clasping, or running. Only humans have hands that are as basic as those of newts and salamanders. And thirdly there are our teeth. As with the hand, dental modifications occurred in almost all mammals, so that carnivores, insectivores, rodents, grazers, etc. all possess their own taxon-specific dental formulae. Once again, only us human beings have teeth that are primitive and unspecialized. It seems as if the cerebral increase was “bought” at the expense of a maturation of other structures, leaving them in an almost foetal, immature state.

With one part of the brain physically over- and outgrowing the more ancient “animal-part” of the brain and then being able to influence and control the evolutionary older brain regions of instincts and automatisms, complete specialization of appendages and other organs, viz. our teeth, seems to have been no longer necessary. In fact, it would have been undesirable and the arrest of maturation at an earlier level in combination with a powerful brain capable of modifying behaviour on the basis of previous experience offered totally new options, unseen in animal evolution up until then. The idea that the human being is a “product of neoteny” (the phenomenon of seeing sexual maturity reached in an otherwise incompletely developed individual with juvenile features) goes back to the Dutch anatomist Louis Bolk in the 20s of the last century. And I think his hypothesis has a lot to speak for it.

After all, was it not also neoteny that was responsible for the evolution of the vertebrates from tadpole-like and actively swimming, tail-bearing larvae of the sessile, filter-feeding sea-squirts? I think so. But it begs the question what a human might have looked like if maturity had not already been reached in a body retaining the juvenile looks. Any idea?

© Dr V.B. Meyer-Rochow and, 2018.
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