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PORTSIDE  February 2012, Week 3

PORTSIDE February 2012, Week 3

Subject:

Oldest Animals Ever Discovered

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Date:

Sun, 19 Feb 2012 22:52:59 -0500

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Oldest Animals Ever Discovered
Posted By: Greg Laden
Smithsonian.com
February 10, 2012
http://blogs.smithsonianmag.com/science/2012/02/oldest-animals-ever-discovered/

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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