VIVENTI, ETÀ dei. — The enormous span of time extending between the solidification of the first terrestrial crust and the Cambrian (the period that opens the Paleozoic era and is already rich in fossil remains) is commonly called the “Archeozoic era” and may be simply subdivided into two periods: the Archean (more ancient) and the Algonkian (more recent).
According to the most modern research based on the radioactivity of rocks, the end of the Archeozoic era dates back approximately 600 million years, and its duration may be estimated at approximately 1,800 million years (v. TERRA, età della). Throughout this period, sediments were deposited (often alternating with eruptive formations) to a total thickness of not less than 30,000 metres. In the more recent part of these rocks, very often profoundly altered by metamorphism, the remains or traces of the most ancient organisms were found, consisting of algae, protozoa, and the most primitive forms of invertebrates; the earliest traces of life are thought to occur at a level corresponding to the Upper Archean.
Almost a century ago, first in Canada and then elsewhere, nodular masses with alternating lamellae of limestone and serpentine, crossed by branching channels, were reported; these curious structures, interpreted by some scholars as giant foraminifers or as calcareous algae, were given the name Eozoon canadense. Today few palaeontologists maintain the organic nature of these structures, since similar ones, evidently resulting from metamorphism, were found in blocks of Jurassic age ejected by Vesuvius. Subsequently, more reliable remains of primitive organisms were discovered; in Finland, certain problematic structures (Corycium aenymaticum) found in Archean rocks were, and still are, interpreted as algae. The “radioactive-lead method” was applied to them, revealing an age of 1,150 million years. This would be the oldest known organism; in any event, in sedimentary rocks of comparable age there are considerable quantities of carbonaceous remains whose presence can be explained only by assuming that they derive from organized beings (probably algae).
Of great interest are the bacteria, now reported from the Archean of various localities. Their activity may perhaps account for the accumulation of politic iron deposits, and there has been little doubt about their nature since it became possible to photograph them in infrared light with sufficient clarity to permit their classification. In the United States, numerous forms collectively called “stromatolites” (Cellenis, Camasia, Vevelandia, Gallatinia, Graysonia) were studied. Almost all were found in Algonkian strata and interpreted as parts of the thallus of algae or also as traces resulting from algal activity; because of the strong analogy between the remains just mentioned and certain present-day calcareous formations, they are believed, in all probability, to be blue-green algae (cyanophytes). Specimens, not always clearly identifiable, of radiolarians and foraminifers, as well as scant remains of invertebrates attributed to sponges, coelenterates (Carelozoon), crinoids, brachiopods (Linguella), and tubes and trails of annelids (Arenicolites, Planolites), molluscs (Hyolites, Chuaria), crustaceans, and arthropods, likewise come from Precambrian strata. Various forms were assigned to the latter, some of them gigantic, found in America and Australia (Beltina, Protadelaidea), partly resembling the Paleozoic eurypterid merostomes; very recent studies, however, suggest that Beltina is nothing more than the impression of an alga, and that the impression of Protadelaidea may have been caused by inorganic agents.
In conclusion, our ideas concerning the first living beings remain rather vague. To explain the great paucity of Precambrian fossil remains (especially of invertebrates), various hypotheses were formulated, some of which apply only to particular localities or do not withstand close scrutiny; the most plausible perhaps consist in supposing that metamorphic processes erased the traces of organisms, or in admitting that the earliest invertebrates were predominantly organisms with little or no mineral skeleton (v. STRATIGRAFIA; TERRA, ETÀ DELLA; UNIVERSO; VITA).