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Human Evolution
Found: One of many missing human links

Researchers working in Ethiopia recently uncovered bones and teeth from one of many previously missing links in the hominid family tree. The newly found remains, researchers say, connect two well-known hominid species that are separated by 1 million years.

Researchers in the Middle Awash region of Ethiopia painstakingly recover bones and teeth of ancient hominids. Copyright 2002 David L. Brill\Brill Atlanta.

Generally speaking, three different phases of human evolution have occurred over the past 6 million years, says Tim White, a paleoanthropologist at the University of California in Berkeley, and a team leader on the new discovery. The past 2 million years “are us and our ancestors, Homo,” he says — marked by a wide geographic range, wide variety in dietary habits, big brains, small teeth and tool usage.

Roughly 2 million years prior, fairly geographically and ecologically widespread, small-brained hominids from the genus Australopithecus walked upright and had relatively larger cheek teeth, White says. And roughly 4 million years ago and earlier, hominids from the group Ardipithecus walked upright, likely had smaller brains and larger teeth, and filled smaller dietary niches and geographic ranges.

Homo is generally thought to have descended in part from Australopithecus, which in turn, is thought to have evolved from Ardipithecus, White says. One problem in truly understanding these evolutionary relationships has long been the missing links between them, however. Nearly 1 million years separate the well-known species Australopithecus afarensis, including the famed Lucy (see Geotimes, May 2001) from the older Ardipithecus ramidus, and the morphological features of the specimens, while similar, did not necessarily support a direct lineage, as White and colleagues reported in the April 13 Nature.

A few years ago, researchers found a possible intermediary species, Australopithecus anamensis, in Kenya, about 1,000 kilometers away from the current dig site. Because it was not in the same region as the other two species, however, it did not help directly support the lineage.

The teeth are from an intermediary species of hominid that links two previously known species in the human family tree. Copyright 2005 Tim D. White\Brill Atlanta.

The newly dated Au. anamensis specimens are about 4.1 million years old and in the same geographical location in successive geologic layers as the other species, thus effectively bridging the gap between Homo ancestor groups, White says. Au. anamensis is the most primitive Australopithecine found to date.

White’s team recovered about 30 teeth and finger, toe and leg bones from eight individuals in a layer of rock in the Afar desert in Ethiopia that they dated to about 4.1 to 4.2 million years ago. The bones and teeth share features of both the older Ar. ramidus and the more recent Au. afarensis and are an intermediary between the two species, both morphologically and temporally.

“We now know where Lucy and her kind [Au. afarensis] came from,” White says, but the research raises further questions as well.

It remains unclear whether or not Ar. ramidus could have been an ancestor to Au. anamensis, as the two species are only separated by 300,000 years or so, a relatively small amount of geological time, White says. “We put forth two hypotheses about the origin of Au. anamensis,” he says: One supports a conventional straight-line evolution from parent to daughter species, and the other is that it was punctuated equilibrium, where there was a quick split from parent to daughter.

“For the moment, it’s really not possible to choose” one hypothesis over the other, says Michel Brunet, a paleoanthropologist at the University of Poitiers in France. Neither hypothesis can be discarded or proven without more fossil evidence to fill in the remaining gaps.

Nevertheless, Brunet says, “this is a very important find and contribution to our knowledge of the human story.”

Megan Sever

Links:
Human Evolution Research Center at Berkeley
"From tree to tumbleweed," Geotimes, May 2001
"New evidence for earliest hominid," Geotimes online, Web Extra, April 15, 2005
"Redating the earliest humans," Geotimes online, Web Extra, Feb. 28, 2005

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