The record for the oldest multicellular life was broken, nay smashed, four years ago with the finding of 2.1 billion year old fossils in Gabon. Understandably, the finders raced to publish before the fossils had been fully described, but a more revealing portrait has now been made public.
Publishing in PloS One a team led by Professor Abderrazak El Albani of the Institut de chimie des milieux et matériaux in Poitiers reveal considerable diversity. The lifeforms are variously circular, elongated and lobed, with some smooth and others folded or knobby. The largest are 17cm across.
Left side shows the stratigraphic surface viewed from above, impressions in the shale are on the right. (A–H) Lobate forms showing sheet-like structure, radial fabric (A, B, G, H) and wrinkled appearance (A).
Although some two billion year old candidates for multicellularity had been found before, prior to the discovery of the Gabonese fossils, the oldest confirmed macroscopic life forms known were 1.6-1.0 billion years old, and multicellular life was thought to have begun with the Ediacaran biota that appeared 575 million years ago.
Consequently, as the PloS paper notes, “Reports of Paleoproterozoic macrofossils tend to be controversial, and considerable uncertainty persists about the nature of such remains.” Nevertheless, the authors maintain that the shapes they have found are fossils that emerged in response to an increase in atmospheric oxygen. Colonies of single celled organisms of even greater age are known, but the paper suggests the Gabonese fossils may represent, “a possible pathway toward the emergence of multicellular macroorganisms.”
“The purported significance of the Gabon fossils is thus not multicellularity as such but the evidence they provide for a first appearance in the fossil record of macroscopic individuality,” the paper says, and a combination of geochemical and morphological evidence presented supports the likelihood that the 400 specimens so far found meet these criteria.
Had oxygen levels remained high, who knows what these species might have become, but instead atmospheric oxygen fell again shortly afterward, and these life forms died out without influencing subsequent evolution. “The emergence and later disappearance of megascopic life in association with oxygen overshoot and fall in the early Paleoproterozoic Eon is consistent with oxygen availability as a driver of evolutionary adaptation, including aspects of body size,” the paper argues.
The fossils are thought to have survived because bacterial activity rapidly replaced their organic material with pyrite. The same shales contain impressions that may have been left by organisms that were not pyritized. The fossils lie beneath the famous Oklo nuclear reactors, where natural conditions allowed sustained nuclear fission for several hundred thousand years.