Major steps in vertebrate evolution.

Abstract

We have come to the end of our story-a long one, covering some half a billion years, it appears. A modern man or other higher vertebrate has traveled far from the simply built insensate type of creature seen in his ultimate metazoan ancestor among the pterobranchs. The course of this evolutionary progression is far from direct and simple, as some might believe to be the case; it is a trail with many twists and turns. Nor is there the slightest reason to attempt a teleological interpretation; there is no trace of design and direction toward an obvious goal. Quite in contrast, it seems clear in many stages of the series that the changes which have taken place are immediately beneficial ones, strongly subject to selection. Obvious, too, is the fact that special environmental factors, biological and physical, have added unexpected quirks to the story. The development of a motile "tadpole" larva at an early chordate stage led to a sharp shift in an evolutionary sequence which otherwise might have simply ended in a sedate filtering form of tunicate type. The development of plant life on the continents opened up to motile chordates a new environment into which few invertebrates could enter and in which the chordates flourished to progress to the vertebrate level. The need for armor as defense against eurypterid enemies appears to have initiated the development of bony skeletal structures, without which the higher vertebrates could never have developed. The widespread late Paleozoic condition of seasonal drought favored progressive developments which, with the attainment of a reptilian stage, had the happy accidental result of the vertebrate conquest of the land, a conquest aided by the emergence of the insects as a basic food supply. The long period of dinosaur dominance seems to have been responsible for the sharpened wits which made the mammalian descendants of the therapsids competent for terrestrial dominance when the reign of the ruling reptiles ended. The arboreal life of primates was finally abandoned by man, but tree-dwelling had endowed his ancestors with advances in brain, eyes, and hands that were highly advantageous when this relatively feeble creature descended to the ground. It has been a long and tortuous journey; but every stage of it shows its effects in the structures and functions of such an end product as ourselves (Fig. 4).