3. Animal life has undergone continuous change since its introduction upon the earth, so that each group of strata, representing a particular geological age, contains a characteristic assemblage of fossil animals.
4. Many of the changes in the history of animals have been progressive or evolutionary, so that strata of early geological time contain distinctly more primitive or lower order forms than the strata of late geological time. But, while the line of evolution has been maintained without a break, culminating in man, there have been many offshoots of a retrogressive nature.
5. Even as far along in geological time as the early Paleozoic era, the highest subkingdom—vertebrates—had no representative whatever. In other words, all the important subdivisions of animal life from a little below fishes to man have been evolved since about the close of the Ordovician period.
6. Any species of animal which ever became extinct has never been known to reappear, and literally tens of thousands of species are known to have become extinct.
7. No species like those now living are found in the more ancient strata, such being confined to the strata of relatively recent geological dates.
8. While more and more highly organized animals have continuously been evolved, many of the earlier and simpler types have persisted, a remarkable case in point being the single-celled animals called foraminifers which may be traced, without very notable change, through the tens of millions of years of geological time from the late Proterozoic era to the present day.
9. Many species have been able to maintain themselves practically without change through long stretches of geological time, while others have had only very brief existence.
When did animal life begin on the earth, and what were the first forms like? We can only partially answer the first question by saying that animals have existed for tens of millions of years, certainly as early at least as Proterozoic time. Up to the present time we are utterly in the dark as to what the earliest animal forms looked like, but we have positive knowledge that the oldest forms found as fossils in the rocks represent creatures which were far more primitive and lower in organization than many animals of to-day, and that since those oldest known forms lived, the animal kingdom has undergone various profound alterations. In view of the above statements, and also the fact that the oldest known plant forms were extremely simple or single-celled, it is more than likely that the first animal life of the earth was single-celled. In harmony with this view is the fact that fossil single-celled animals are found in the very oldest (Proterozoic) rocks which contain any definitely determinable fossil animals.
Do the most ancient known rocks show that animal life existed during Archeozoic time? In the preceding chapter we pointed out the fact that the carbon (in the form of graphite), so commonly present in those most ancient known strata, proves the existence of life of some kind during Archeozoic time. But because nothing like definitely determinable fossil forms have thus far been discovered in those rocks, we cannot be sure whether the carbon represents plant or animal life or both, though certainly plants of very low order at least must have existed. Because of the intense alteration (metamorphism) of those very old strata, all definite forms have long since been obliterated as such. We may, however, in the light of the vast evolution which took place through succeeding geological time, be very sure that any animals which may have existed during Archeozoic time were in general much simpler forms than those of even early Paleozoic time.