The remains of Omo I and Omo II were buried in the lowest sediment layer, called Member 1, of the 330-foot-thick (100-meter-thick) Kibish rock formation near the Omo River.
In addition to GPS, more advanced dating techniques have also been developed. The researchers sampled the volcanic ash on both sides of the river that lay above where the fossils were found. The ash was the same on both sides.
"Then we had to find something to date, and what that takes is a lot of walking," Brown said. "Most of the ashes are very fine grained, they dont have pumice [fragments] in them, so you go along and you go along, and eventually you find a place where there are pumices."
The presence of feldspar crystals from a volcanic eruption inside pumice fragments is an indication that the crystals have not been contaminated. Such unadulterated crystals can be dated using a technique called potassium-argon dating.
"By dating the crystals held in the pumice, you can say with a high level of confidence that everything in that member [group of sediment layers] is nearly the same date," Brown said. "We used a dating technique called 40AR/39AR, which is a variant of potassium-argon dating."
In the same Member 1 sediment layers, the team found additional Omo I bones, animal fossils, and stone tools.
The work was funded by the National Geographic Society, the National Science Foundation, the L.S.B. Leakey Foundation, and the Australian National University.
Widening the Gap
Although both Omo I and Omo II were classified as Homo sapiens in 1967, the Omo II remains were considered much more primitive. Finding that the two individuals lived at around the same time in the same location suggests that, when modern humans first appeared, there were other, less modern populations also on the scene. The finding may add some new perspective to how we think about how and when "modern" human anatomy evolved.
"I have previously regarded Omo II as an archaic or primitive H. sapiens and Omo I as a modern H. sapiens, which would make them the same species," Stringer said. "If Omo I and II do belong together, the variation in the population is greater than I expected, but given what we see in larger fossil samples from other regions, we may need to accept that African populations showed large [physical-form] variation at this time."
Everyone agrees that the Omo II cranium is more primitive than the Omo I skull in many features, Fleagle said.
"Some see the two as part of a continuum, others see them as very distinct types of hominid," he said. "Whether Omo II gets put in Homo sapiens depends upon where one draws the boundary between H. sapiens and whatever species comes beforeH. ergaster, H. erectus, H. heidelbergensis.
"Regardless of how Omo II is classified, " he continued, "I don't consider it surprising to find two different morphologies existing at the same time. We know that Homo sapiens and Neandertals existed in Europe at the same time and that in the early Pleistocene [epoch] there was diversity of early hominid morphologies [or body forms]. Indeed, virtually every site that has early modern humans ... seems to show a diversity of morphologies with some more modern and some less so."
Exactly when modern behavior, as opposed to modern anatomy, emergedindeed even how to define modern behavioris another area in which the Omo fossils might contribute some insight. Common elements used to define modern behavior include planning ahead; innovating technologically; establishing social and trade networks; adapting to changing conditions and environments; and exhibiting symbolic behavior like cave painting, beadmaking (used to show status or group identity), or burying the dead.
The crux of the argument comes down to whether these abilities resulted from a sudden biological and genetic revolution or from a more gradual evolution of abilities that culminated around 50,000 years ago.
"I think we are still determining when "modern" behavior started to evolve, and my guess is that it too will have deeper roots in Africa," Stringer said. "There is growing evidence that elements of modern behavior were there a hundred thousand years ago, and I think the gap or mismatch between the emergence of modern anatomy and modern behavior may well be much less significant than currently believed."
Spencer Wells is a geneticist and an anthropologist and a National Geographic Emerging Explorer. From an analysis of DNA of thousands of men around the world, Wells says he has discovered that all humans alive today can be traced back to a small tribe of hunter-gatherers who lived in Africa 60,000 years ago.
"Many anthropologists, myself included, believe that what makes us truly human is our modern behavior, enabled by a modern brain," Wells said. "Modern behavior starts to show up sporadically around 70,000 to 80,000 years ago but doesn't really take off until around 50,000 years agothe "Great Leap Forward" and dawn of the Upper Paleolithic [early Stone Age]."
The human population appears to have crashed to around 2,000 individuals around 70,000 years ago, at the same time they were headed into the worst part of the last ice age. The crash was possibly brought on by a massive volcanic eruption, Wells said.
"The hypothesis is that the survivors of this near-extinction event had to be smarter in order to survive, and this allowed them to settle the rest of the world outside of Africa. So, 'human-ness' may not been widespread until around 50,000 to 60,000 years ago, and this could be seen as the real origin of our species."
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