Evolution and a Succession of Forms

　
In giving introductory courses in the life sciences, which are naturally presented from the evolutionary point of view, university professors often refer to the "Evolutionary Base Line." By this is meant a succession of forms of life which are chosen to represent what are believed to be the basic steps in the process of development from amoeba to man.

This base line begins with unicellular forms such as Paramecia, but it is worth noticing that an authority like Gaylord Simpson considers the step from no life to amoeba as being as great as the whole passage from amoeba to man. In order to make the picture more or less complete, the Evolutionary Base Line will be given very briefly with a note explaining the advance represented by each form in the series.

The line is traced from the invertebrates to the vertebrates through Amphioxus, a creature which stands lowest in the phylum Chordata. This creature lacks a brain in the accepted sense, all its controls being distributed along the dorsal cord. It differs from other invertebrates in the possession of a pharynx perforated by gill slits which, however, serve to strain food rather than to provide an oxygen supply. Next in this base line it is customary to place the Lamprey, which has its notochord apparently encased in part in bony segments. Lampreys have "nostrils," rudimentary eyes, ears under the skin, and a single nasal pouch. These animals have a brain, but no movable jaw and no fins. Next comes Climatius which, like the former, is quite small but now has a kind of primitive jaw. Then comes Cladoselache, with paired fins, fully developed jaws, jointed and muscled, and with both upper and lower segments free to move. We are then introduced to Crossopterygians. Here we find the presence of bony structures underlying the fins constituting genuine limbs. These are viewed as the forerunners of legs in amphibians. Unlike the animals which preceded, they possessed a simple lung. Associated with these representative creatures in the base line are the Dipnoi, or lung fishes, which are at a similar level of development except that they have a modified mouth construction in which there is an opening from the nasal sac (now a kind of lung) to the inside of the mouth. This enabled the animal for a limited time to dispense with the use of the gills and use oxygen stored in the nasal sac. In conjunction with the development of fins with a bone sub-structure, it became possible for these creatures to make short trips out of the water. It is generally held that, in Devonian times, extended drought dried up many rivers leaving only stagnant pools.

When these became depleted of oxygen, the lung fishes were able to move to new pools. Thus began, supposedly, the conquest of the land.

The most primitive amphibians appear in the Carboniferous Era, namely, the labyrinthodonts, in which appear rudimentary legs with five webbed digits and a more highly developed olfactory sense. Later in the Carboniferous Era, the Cotylosaurs are first discovered. One particular species, the Seymouria, have genuine legs which, however, were very squat and suggest that the possessor was not really at home on land. Nevertheless, the Seymouria produced the first land-type eggs in which the water environment is contained within the egg and surrounded by a "shell."

Up to this point, all living creatures were cold-blooded. So long as they stayed in the water, this was no disadvantage, a water environment being probably the most stable environment conceivable. In very hot weather, the deeper waters remain cool and in sub-zero weather, the ice which forms on the top protects the water beneath from dropping too low. It is generally thought that this accounts for the persistence of many fishes, such as Coelacanths, through millions and millions of years with virtually no change in form. However, when the environment over land areas had been prepared by plant life and the atmosphere had been made fit for animals that breathe air, it was then possible for living creatures to leave their watery home and come up to stay on the dry land. At first these had been forays only, then some of the first pioneers began to lay their eggs on the land, though they and the young returned to the water as a natural habitat. Longer and longer stays were made by such creatures in this new environment, but so long as they were cold-blooded, they were at the mercy of the elements, for when the sun went down or cold weather came, their energy was reduced to a very low level.

There is an early creature occasionally mentioned in the Evolutionary Base Line, the Dmetrodon, that was apparently supplied with a strange sail-like structure along its back supported by a number of "masts." Sometimes it looks as though God took delight in experimenting. It is not certain what purpose these served, but it has been thought that they were heat exchangers, a kind of network of arteries set in a thin membrane stretched out to present the maximum surface area to the sun, thus providing the animal with heat more rapidly than otherwise. Presumably, when cold temperatures came, the reverse process was avoided by a form of vasoconstriction so that deep body heat was not lost to the cool air through the heat exchangers. But no cold-blooded animal can remain active and alert outside of a fairly narrow range of temperature variation. Such creatures could never, therefore, become complete masters of the environment.

At this point, it is customary to follow a sideroad in the Evolutionary Base Line. Warm-blooded animals as a class bear their young alive. The egg, as such, is never "laid." But there was a stage ‹ and representatives still exist ‹ in which the transition was not completely made, as represented by the Duckbilled Platypus and the Spiny Anteater, both of which are egg-laying mammals. These creatures are truly mammals and warm-blooded, yet the young they bear appear first as reptilian eggs. A further stage is represented by the Marsupials. These are pouched animals which, like the Duckbilled Platypus and Spiny Anteater, are not included in the base line; but they are also referred to as transitional forms, because although the egg is never laid in this case, the young are nevertheless hatched "too soon" and have therefore to be returned to a kind of "shell" to complete the full process of embryonic development. They are not included because they are too "late" in the series.

Transitional forms between cold-blooded animals and mammals are sometimes represented by the Therapsids, of which one particular species, the Cynognathus, is customarily singled out. It is not necessary here to detail the reasons why this species is considered transitional, except to mention that it has a number of mammalian features, especially in the mouth region. And although it is coldblooded, it appears to have been highly active -- which most coldblooded land animals are not.

Among the earliest representatives of the Placentals, which bear their young in a fully developed state are Hedgehogs, Ground Shrews, and Tree Shrews. The Evolutionary Base Line is traced through the Tree Shrews for several reasons, one of which is its completely unspecialized form. These little animals enjoy the privilege of being able to sit upright and use prehensile front feet to feed. While they have an exceedingly simple brain, they are highly mobile, with limbs capable of complex movements and a fair range of stereoscopic vision.

On a rung nearer the top of the ladder are placed the Lemurs, in which there is a development of fingernails and well-formed "hands," with opposable thumbs widely separated and separable from the rest of the fingers. They appear to have a wide range of vocalization and their eyes are closer to the front than similar mammals which preceded them. They were quite common in Eocene times. Their facial appearance was somewhat foxlike, a fact which has led some authorities to question whether they should be classed as Primates.

Soon after the appearance of the Lemurs, we find the Tarsioids. These creatures are placed much higher in the scale of evolution and are classed as Primates. Their eyes are turned completely forward, the nose is very short, and the flattening of the face is taken to mean that the sense of smell has become less important and stereoscopic vision has now taken over.

To be continued....