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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.
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 [that] anamensis is sandwiched between earlier
and later hominids is what is really significant about this
Ethiopian sequence," said Tim White.
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.
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."
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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