Posted on 03/18/2008 7:11:34 AM PDT by blam
Skull Changes Show Time of Human-Neandertal Split
Scott Norris
for National Geographic News
March 17, 2008
Gradual changes in human skull size and shape suggest a split between humans and Neandertals (often spelled Neanderthals) about 300,000 to 400,000 years ago, according to a new study.
The work provides the first estimate a divergence date for modern humans and Neandertals based on the rate of change of physical characteristics.
Genetic Drift
Just as DNA changes accumulate over time and provide a kind of "molecular clock" by which the separation of closely related species can be dated, evolved differences in physical form can provide similar information, researchers say.
But that is true only if the differences are due to the random process of "genetic drift," and not driven by natural selection, said study lead author Tim Weaver of the University of California Davis.
During genetic drift, different traits accumulate in separate populations by the spread of chance mutations—not because the traits provide any individual advantage in survival or reproduction.
The new study builds on previous work by Weaver's team suggesting that such random genetic changes are the reason people no longer sport the low forehead and protruding brow of our Neandertal relatives.
If differences in human skulls are due to genetic drift, Weaver said, "then the amount of divergence will be proportional to the amount of time elapsed since the ancestors of Neandertals and modern humans [separated] from each other."
The study by Weaver's team appears this week in the journal Proceedings of the National Academy of Sciences.
The new study is based on a series of 37 measurements of the modern human and Neandertal skulls.
For instance, researchers studied the width of the jaw and eye sockets and the distance between the various bones that make up the cranium.
Different human populations today have, on average, slight differences in cranial features. Our heads are not all shaped exactly the same—and neither are our genes.
Scientists can use changes in DNA sequences to estimate the amount of time that has passed since present-day human populations began to diverge.
With this information, Weaver's team was able to measure the rate at which changes in skull form have occurred.
Knowing that rate allowed the researchers to plot the differences between humans and Neandertals backward in time and determine when the two groups separated from one another.
"The striking finding is that we obtained such similar dates to those from DNA sequences by assuming that [skull form] diverged by genetic drift," Weaver said.
Erik Trinkaus is a Neandertal expert at Washington University in St. Louis.
The new study by Weaver's team "is valid in indicating that those aspects of the human cranium that are likely to be governed by random processes, such as drift, are in agreement with … genetic analyses," he said.
"Both of them are [also] in general agreement with the fossil record, which indicates that you start getting divergent aspects of human anatomy in Africa and Europe 300,000 to 500,000 years ago."
Not so Random
Not all changes in the human skull have been random.
The large heads and brains of both humans and Neandertals result from natural selection that occurred earlier in our evolutionary history, experts say.
Once these features evolved, however, the main role of natural selection on the skull has been to constrain its dimensions to a range of variation necessary to accommodate and protect the brain, Weaver said.
"Within this range, cranial form may have virtually no influence on which individuals survive and reproduce," he noted.
"Under these circumstances, changes in cranial form will be due to chance."
“Ask Job...”
I don’t think you understand Job.
No. I don’t.
It’s worth the effort.
Don’t think I haven’t tried!
Took me decades, and I still don’t have it completely. IMO, one must return to older concepts of virtue and value.
Tight-fitting genes...a la Mac Davis....
False. The hominid age estimates are based on many different lines of evidence. If you studied them, instead of just denying them, you would have a more valuable opinion on the subject.
RC dating even in theory is only good to around 50K years and the decay methods you read about for much older things don’t apply to hominid remains.
I know the range of radiocarbon dating; I do a lot of radiocarbon dates in my work. And the "decay methods" for older things do apply directly to hominid remains. If you date the volcanic layers above and below a specimen, you know a lot about the age of that specimen. Try the following paper:
Radiometric Dating: A Christian Perspective by Dr. Roger C. Wiens.Aside from everything else Gunnar Heinsohn has demonstrated that the stratigraphical basis for some of the age estimates involving neanderthal remains in caves are totally fubar and that there is no defensible basis for assigning more than about 100 years to a layering system which is normally assumed to involve about 60,000 years; that would be in “Wie Alt Ist Das Menschengeschlect?” The counts of tools and paraphernalia corresponds to about 100 years and not 60,000.
From what I can see on the web Heinsohn is a crank. Why should anyone believe anything he says when the vast mountains of evidence show otherwise?
Face it, radiometric dating and several other lines of evidence all point in the same direction. Those who deny these dating methods need to come up with something better than Heinsohn or Velikovsky.
ping
The issue is confused enough already without gratituitous confusations.
Flats or heels?
You might like the vodka martini chilled with foam of tunneled sub_quark particles...
Jump in. The waters fine :>)
Thanks for the ping!
Radiocarbon anomolies.
I have just finished reading The Cycle of Cosmic Catastrophes that one of you inspired me to get. This book hypothesizes that the earth was first bombarded by radiation from a realatively nearby supernova about 41 Kya. Then a secondary wave of cosmic effects hit about 34 Kya. Finally, between 16 and 13 Kya there was an influx of cometary debris and comets disturbed in orbit or emanateing from the supernova direction which caused major catastrophe in the Northern Hemisphere especially North America. This occurred at the time of the thriving Clovis culture. This culture disappeared at the same time as most large mammals in the northern areas. There are large peaks of radiation from various elements at that layer, well dated by Clovis cultural and human remains. In particular Figure 2.2 states, “About 41,000 calendar years ago, the 14C in Earth’s atmosphere increased dramatically. Since then, it has been mostly in a gradual decline.” I would think this would cause a number of problems for carbon dating in the past 50 thousand years.
I have just finished reading The Cycle of Cosmic Catastrophes that one of you inspired me to get. This book hypothesizes that the earth was first bombarded by radiation from a realatively nearby supernova about 41 Kya. Then a secondary wave of cosmic effects hit about 34 Kya. Finally, between 16 and 13 Kya there was an influx of cometary debris and comets disturbed in orbit or emanateing from the supernova direction which caused major catastrophe in the Northern Hemisphere especially North America. This occurred at the time of the thriving Clovis culture. This culture disappeared at the same time as most large mammals in the northern areas. There are large peaks of radiation from various elements at that layer, well dated by Clovis cultural and human remains. In particular Figure 2.2 states, “About 41,000 calendar years ago, the 14C in Earth’s atmosphere increased dramatically. Since then, it has been mostly in a gradual decline.” I would think this would cause a number of problems for carbon dating in the past 50 thousand years.
Atmospheric fluctuations in the levels of 14C certainly would be a problem for accurate radiocarbon dating. This was recognized by deVries (1958) and others, and a calibration curve based on tree ring dating and other criteria has been established to correct for those fluctuations.
The calibration curve is not yet to the full 50,000 years covered by radiocarbon dating, but the time period up to about 20,000-25,000 is pretty well covered. The period up to about 12,600 years is particularly well covered using the bristlecone pines from the White Mountains of southern California. I think that coverage is in 10-year increments.
There are other potential sources of error in radiocarbon dating that have also been taken into account.
Since I haven't posted it lately, here is my list of radiocarbon links:
ReligiousTolerance.org Carbon-14 Dating (C-14): Beliefs of New-Earth CreationistsRadiometric Dating: A Christian Perspective by Dr. Roger C. Wiens.
This site, BiblicalChronologist.org has a series of good articles on radiocarbon dating.
Are tree-ring chronologies reliable? (The Biblical Chronologist, Vol. 5, No. 1)
Tree Ring and C14 DatingHow does the radiocarbon dating method work? (The Biblical Chronologist, Vol. 5, No. 1)
How precise is radiocarbon dating?
Is radiocarbon dating based on assumptions?
Has radiocarbon dating been invalidated by unreasonable results?
Radiocarbon WEB-info Radiocarbon Laboratory, University of Waikato, New Zealand.
It was Sunkenciv...he got me to buy/read it too.
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