Convoluted human evolution: H. naledi branch; Dates? (Introduction)

by David Turell @, Tuesday, July 12, 2016, 14:33 (1105 days ago) @ dhw

Dating problems plague this discovery in a cave:

"Last September, scientists announced the discovery of a never-before-seen human relative (hominin), now known as Homo naledi, deep in a South African cave. The site yielded more than 1,500 bone fragments, an astonishing number in a field that often celebrates the identification of a single tooth. That rich fossil cache revealed much about the creatures, yet it left one glaring question unanswered: when did Homo naledi live? The scientists had no evidence for how old the fossils were. Without that information, it was very hard to know where the new species fits on the tangled human family tree, and to figure out its true meaning.


"Over the past century, anthropology's situation improved greatly with the introduction of new techniques, most notably the measurement of radioactive isotopes that decay predictably over time. Radiocarbon dating, which examines the carbon isotopes within the fossils, can measure the age of the bones themselves. Other techniques, such as potassium-argon dating, can derive the age of surrounding volcanic rock. But even those methods were (and are) often limited. Radiocarbon dating works only on fossils 50,000 years or younger, which is not helpful for most of the 7 million years or so of human evolution. Potassium-argon dating can be applied to much older fossils, but it is useless where there are no volcanic rocks. There have also been vexing problems of technique.


"The lack of age information is especially confounding because Homo naledi contains such an odd mixture of morphologies. Some of the fossils' traits look very modern, for example their human-like hands and feet; others look remarkably ancient, for instance their primitive shoulders and hips. The evolutionary implications of naledi would look entirely different if the fossils were 2 million versus 20,000 years old - and either is possible. A creature with a modern, delicate hand resembling those of modern humans would present a baffling surprise if it lived 2 million years ago. Conversely, a primitive shoulder that appears to be built for climbing would make sense millions of years ago, but doesn't fit with our ideas about hominin lifestyles in the more recent past. One scientist went so far as to claim that without a convincing measured age, the naledi fossils reveal almost nothing about human evolution.


"On the other hand, the story of the Neanderthal shows how far we've come. Using radiocarbon dating, we now know that Neanderthals lived from about 400,000 years ago to 40,000 years ago. Throughout the 20th century, scientists refined these dates using increasingly creative techniques, such as measuring light produced by heated crystals to derive their precise compositions. This knowledge has allowed us to prove that Neanderthals preceded but overlapped with (and occasionally interbred with) modern Homo sapiens.

"While they wait for similar dating breakthroughs for naledi, some scientists see the lack of an established age as an opportunity in disguise. It allows them to focus on the fossil's anatomy without being biased by information about its chronology. Anthropologists have collected vast numbers of fossils since that first find in 1856. They don't need to study naledi in isolation; they can compare it - statistically, morphologically, visually - to other known hominins."

Comment: Tony's complaint about dating is vindicated. Anatomic comparisons with known dated fossils should help.

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