Fossils fill gap in human lineage

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Apologies if this has been seen already
Iain

http://news.bbc.co.uk/2/hi/science/nature/4900946.stm
Fossils fill gap in human lineage
By Paul Rincon
BBC News science reporter

Australopithecus anamensis fossils from Asa Issie Photo © 2005 Tim
D. White\Brill Atlanta
The finds include teeth, hand and foot bones, and a thigh bone
(Image: ©Tim D White\Brill Atlanta)
Fossil hunters have found remains of a probable direct ancestor of
humans that lived more than four million years ago.

The specimens of this ancient creature are helping bridge a long
gap during a crucial phase of human evolution.

Professor Tim White of the University of California, Berkeley, and
colleagues unearthed the cache of fossils in the Middle Awash
region of Ethiopia.

They describe the finds, which belong to the species
Australopithecus anamensis, in the journal Nature.

Australopithecus is an important ancient genus of humanlike
creatures, or hominids.

I think you could argue...what we're monitoring here is the genesis
of that second stage of human evolution - the genesis of
Australopithecus
Tim White, UC Berkeley
Our own genus, Homo, is widely thought to have evolved from this
group. So the relationship of Australopithecus to even earlier
bipedal hominids is crucial to understanding where we all
ultimately come from.

When placed together with other fossils from the same general area
of Ethiopia, the 4.1-million-year-old anamensis specimens appear to
establish an evolutionary succession between earlier and later
species.

"The fact anamensis is sandwiched between earlier and later
hominids is what is really significant about this Ethiopian
sequence," Tim White told the BBC News website.

Middle man

The finds close the gap between a more ancient species known as
Ardipithecus ramidus, which is found at 4.4 million years and a
later species known as Australopithecus afarensis, which is present
in the Middle Awash 3.4 million years ago.

Australopithecus anamensis is intermediate between the two not only
chronologically but also in terms of its anatomy.

Photo © 2002 David L. Brill\Brill Atlanta
The Middle Awash region of Ethiopia has yielded many important
finds (Image: ©David L Brill\Brill Atlanta)
The anamensis species is not new, but, say the researchers, "this
is the first time that these three species have been shown to be
time-successive in a single place".

One explanation is that one species simply evolved into the other -
so-called phyletic evolution.

Another possibility is that Australopithecus first emerged as a
side branch of Ardipithecus. Under this scheme the mother species
would have lived alongside the daughter species for some period of
time before the mother species died out.

But no overlap between any of the three species has been found in
Ethiopia.

Mind the gap

"I think you could argue, fairly, that the circumstantial evidence
based on geography and habitat is of one evolving phyletically into
the other and what we're monitoring here is the genesis of that
second stage of human evolution - the genesis of Australopithecus,"
White explained.

But, he added: "We cannot disprove the alternative hypothesis just
yet."

Photo © 2005 Tim D. White\Brill Atlanta
The finds suggest an evolutionary succession in Ethiopia (Image:
©Tim D White\Brill Atlanta)
The new discoveries go some way to bridging the gap between
Ardipithecus and Australopithecus, but do not entirely plug it.

"The gaps don't get entirely filled; you fill a big gap and create
two smaller ones," said Professor White.

"Now we're looking at a gap between 4.4 million and 4.1 million.
That's 300,000 years; an awful lot of time when measured on a human
timescale, but not that long on a geological one."

The fossils represent at least eight individuals and include the
largest hominid canine ever found, the earliest known
Australopithecus thigh bone as well as hand and foot bones.

In the woods

The excavation at Asa Issie also uncovered the remains of pigs,
monkeys and big cats. The fauna suggest that anamensis was living
in a closed, wooded habitat.

Australopithecus anamensis had a significantly thicker layer of
enamel on its teeth than Ardipithecus, suggesting the later hominid
was adapting to eating a more abrasive diet of roots.

In many species, this is a fallback food when resources are scarce,
but it is not clear what caused the diet shift in this case.

The Turkana Basin in Kenya has also yielded Australopithecus
anamensis fossils.

Australopithecus afarensis was first recognised in the 1970s on the
basis of the now famous "Lucy" skeleton from Hadar, Ethiopia, and
footprints preserved in volcanic ash at Laetoli, Tanzania.

Tim White, Gen Suwa and Berhane Asfaw discovered the first
Ardipithecus ramidus fossils in the 1990s.

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Received on Wed Apr 12 19:24:41 2006