LIVING BEINGS, AGE OF. — The enormous span of time that elapsed between the solidification of the Earth’s first crust and the Cambrian (the period that opens the Paleozoic Era and already abounds in fossil remains) is commonly referred to as the “Archeozoic Era” and may be simply divided into two periods: the Archean (older) and the Algonkian (younger).
According to the most recent research based on the radioactivity of rocks, the end of the Archeozoic Era dates back to approximately 600 million years ago, and its duration can be estimated at roughly 1,800 million years (v. EARTH, AGE OF). Throughout this time, sediments were deposited (often alternating with igneous formations) to a total thickness of no less than 30,000 meters. In the more recent portion of these rocks, which have often been profoundly altered by metamorphism, remains or traces of the most ancient organisms have been found, consisting of algae, protozoa, and the most primitive forms of invertebrates; the earliest traces of life appear to be at a level corresponding to the Upper Archean.
Nearly a century ago, nodular masses with alternating laminae of limestone and serpentine, traversed by branching channels, were reported first in Canada and later elsewhere. These curious structures, interpreted by some scholars as giant foraminifera or calcareous algae, were named *Eozoon canadense*; today, few paleontologists uphold the organic nature of such structures, since similar ones—clearly due to metamorphism—have been found in Jurassic blocks ejected by Vesuvius. Subsequently, more reliable remains of primitive organisms were discovered; in Finland, certain problematic structures (Corycium aenigmaticum) found in Archean rocks have been interpreted as algae, subjected to the “radioactive lead method,” which revealed an age of 1,150 million years. This would be the oldest known organism; and in any case, in sedimentary rocks of similar age, there are notable quantities of carbonaceous remains whose presence can only be explained by assuming they derive from organized beings (probably algae).
Of great interest are the *bati*, now reported in the Archean of various localities, whose action may account for the accumulation of iron deposits; doubts about their nature were dispelled once they could be photographed in infrared light with such clarity as to permit classification. In the United States, numerous forms collectively called “stromatolites” (Collenia, Camasia, Neuclandia, Gallatinia, Graysonia) have been studied, found almost entirely in Algonkian terrains and interpreted as parts of algal thalli or as traces produced by algae; due to the strong similarity between these remains and certain modern calcareous formations, they are most likely blue-green algae (Cyanophyceae). From Precambrian terrains also come specimens—not always clearly identifiable—of radiolarians and foraminifera, as well as scarce remains of invertebrates assigned to sponges (Carelozoon), crinoids, brachiopods (Lingulella), annelid tubes and trails (Arenicolites, Planolites), mollusks (Hylites, Chuaria), crustaceans, and arthropods. To the latter group various forms have been referred, some of gigantic size, found in America and Australia (Beltina, Protadelaiidea), partly resembling Paleozoic eurypterid merostomes; however, very recent studies suggest that Beltina may simply be the imprint of an alga, and that the impression of Protadelaiidea could be due to inorganic agents.
Of the earliest living beings, we have only rather vague ideas. To explain the great scarcity of Precambrian fossil remains (especially of invertebrates), various hypotheses have been proposed, some of which hold value only for specific localities or fail under close scrutiny; the most plausible perhaps are that metamorphic actions erased the traces of organisms, or that the first invertebrates were predominantly soft-bodied or lacked mineralized skeletons (v. STRATIGRAPHY; EARTH, AGE OF; UNIVERSE; LIFE).