VIVENTIAN AGE. - The enormous span of time that elapses between the solidification of the first terrestrial crust and the Cambrian (the period that opens the Paleozoic era and that is already rich in fossil remains) commonly takes the name of "Archeozoic era" and can simply be divided into two periods: Archean (older) and Algonkian (more recent).
According to the most modern research based on the radioactivity of rocks, the end of the Archeozoic era dates back to about 600 million years ago, and its duration can be estimated approximately at 1800 million years (v.
TERRA, età della). During all this time sediments were deposited (often alternating with eruptive formations) to a total thickness of no less than 30,000 meters. In the more recent part of these rocks, very often profoundly altered by metamorphism, were found the remains or traces of the most ancient organisms, consisting of algae, of protozoa and of the most primitive forms of invertebrates; the first traces of life would be found at a level corresponding to the upper Archean.
Now, almost a century ago, there were reported, first in Canada and then elsewhere, nodular masses with alternating lamellae of limestone and serpentine, traversed by branching canals; to these curious structures, interpreted by some scholars as giant foraminifera or as calcareous algae, was given the name Eozoon canadense; today few paleontologists maintain the organic nature of such structures, since similar ones were found, evidently due to metamorphism, in blocks of Jurassic age ejected from Vesuvius. Subsequently more reliable remains of primitive organisms were found; in Finland certain problematic structures (Corycium aenymaticum) found in Archean rocks were, and still are, interpreted as algae, to which the "radioactive lead method" was applied and which thus revealed an age of 1150 million years. This would be the most ancient known organism; and in any case in sedimentary rocks of analogous age there are notable quantities of carbonaceous remains whose presence can be explained only by admitting that they derive from organized beings (probably algae).
Of great interest are the bacteria, by now reported in the Archean of various localities, to whose action is perhaps due the accumulation of oolitic iron deposits and about whose nature few doubts are entertained since it became possible to photograph them in infrared light with such clarity as to permit their classification. In the United States numerous forms were studied, called collectively "stromatolites" (Cellenis, Camasia, Vevelandia, Gallatinia, Graysonia), found almost all in Algonkian terrains and interpreted as parts of the thallus of algae or also as traces due to the action of algae; because of the strong analogy between the remains just cited and certain present-day calcareous formations, it is believed to be in all probability a matter of blue-green algae (cyanophyceae). Also from Precambrian terrains come specimens, not always clearly identifiable, of radiolarians and of foraminifera, and scanty remains of invertebrates ascribed to sponges, coelenterates (Carelozoon), crinoids, brachiopods (Linguella), tubes and trails of annelids (Arenicolites, Planolites), mollusks (Hyolites, Chuaria), crustaceans, arthropods. To the latter were referred various forms, some of them gigantic, found in America and in Australia (Beltina, Protadelaidea), partly similar to the eurypterid merostomes of the Paleozoic; very recent studies, however, suggest that Beltina is nothing other than the imprint of an alga, and that the impression of Protadelaidea may be due to inorganic agents.
In conclusion, our ideas about the first living beings are rather vague. To explain the great poverty of Precambrian fossil remains (especially of invertebrates), various hypotheses were formulated, some of which have value only for particular localities or do not withstand acute criticism; the most plausible perhaps consist in holding that metamorphic actions erased the traces of organisms, or in admitting that the first invertebrates were predominantly organisms poor in or devoid of mineral skeleton (v. STRATIGRAFIA; TERRA, ETÀ DELLA; UNIVERSO; VITA).