February 10, 2012
Life on Earth began very soon after the planet formed about 4.5 billion years ago. After the planet cooled enough, perhaps only tens of millions or a few hundred million years passed before the first life-like things existed. The geological record is sparse for that early period, about 3.5 billion to 3.8 billion years ago, so we can’t be sure. (And it may depend in part on how one defines “life.”) For the next few billion years, it seems, single-celled organisms had the run of the planet. Eventually, multi-celled organisms with differentiated tissue—like organs or other body parts—evolved from a subset of these single-celled forms. It is tempting to assume that the earliest multi-celled organisms were colonies of similar cells somehow functioning together (lots of life forms like this exist today), and that differentiation into different kinds of tissues evolved from this relationship, but the direct evidence to demonstrate this in the fossil record does not yet exist.
For years and years, the earliest evidence of early “metazoans” (animals with body parts) hovered around a half a billion years ago, but over time, earlier and earlier finds were made and the oldest date of something we would recognize as an animal was pushed back in time to about 650 million years ago. Now a new discovery pushes that date back.
A team of researchers based in Namibia, South Africa, Australia and the United Kingdom now report fossils from a Namibian deposit that seem to be animals and apparently date to about 760 million years ago. This extends the known time span of animals on the planet by about 17 percent.
The organisms are named Otavia antiqua and were like sponges. The genus name comes from the rock formation in which they were found, the Otavi Group, though they are also found elsewhere in Namibia. The rock formations in which they are found are several kilometers thick and were deposited in shallow marine environments on an ancient continental shelf. The deposits include volcanic rocks that are dated with a very accurate “parent-daughter” technique using uranium and lead. The dates are further verified by the stratigraphic location of the deposits beneath rocks dated to about 635 million years ago.
Otavia antiqua vary in size from less than half a millimeter to about 5 millimeters (about one hundredth of an inch to 2 tenths of an inch) and are globular or oval in shape. They have little holes on their surfaces, some of which lead to passageways to an internal cavity.
It appears that Otavia antiqua deposited a matrix of minerals between cells to develop a structure, and had a hollow area inside which has subsequently been filled with sediment in the fossils. Presumably, this small creature fed by passively capturing and processing microbes that passed through the holes and into the opening.
Based on the nature of the sediment inside and outside the fossils, the research team thinks that some of the fossils were redeposited from their original location while others are found today in situ, meaning that the fossil is found in the location in which it originally lived (and died).
There are several alternative explanations for these fossils: They could be some form of stromatolite (a bacterial biofilm), or a cluster of plankton-like organisms. However, the researchers make a good case that none of the alternative explanations are likely.
The most striking thing about Otavia antiqua might be the longevity of the life form. It is found throughout ancient sediments that span about 200 million years. If this in fact represents the range of time over which Otavia antiqua existed, without changing, then we should be impressed. This time period spans the hypothesized “Snowball Earth” period during which the entire planet became frozen, or nearly so, multiple times.
I recommend printing out the picture of Otavia antiqua and framing it. You can hang it on the wall and tell people it is your distant ancestor! (Though in all likelihood a cousin and not a grandparent.)
Brain, C., Prave, A., Hoffmann, K., Fallick, A., Botha, A., Herd, D., Sturrock, C., Young, I., Condon, D., & Allison, S. (2012). The first animals: ca. 760-million-year-old sponge-like fossils from Namibia South African Journal of Science, 108 (1/2) DOI: 10.4102/sajs.v108i1/2.658
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