On Fri, 19 Sep 2003, Barry Groves wrote:
> Bob Avery said
> >
> > Is there any proof that all or most of the Earth was ever covered in ice?
In short: No.
> Is there any doubt? When there was so much water stored in ice that sea
> levels were low enough for animals and humans to walk between what is now
> mainland Europe and the British Isles, then I submit that the whole globe
> must have been considerably cooler than it is today.
Major glaciations have occurred during the Pleistocene (1.8 million to
10,000 years ago), before that there were cyclical temperature variations
but any glaciations were much smaller. During the Pleistocene the
majority of the earth was never covered in ice. Ice sheets and glabal
temperatures fluctuated, with some periods actually warmer than the
present. At their maximum extent ice sheets extended as far south as the
river Thames and northern Germany in Europe, with additional major ice
caps in the Alps, southern Andes and parts of East Asia.
At the last glacial maximum (LGM) global average temperature was about 5
degrees C below today, but there were massive variations in this decrease
across the globe:
Zone greatest decrease in mean temperature (oC)
northern temperate 7
Equatorial 3
Southern temperate 3
There were of course local variations on the averages, with parts of
Britain 20 degrees below today's average.
(Mostly summarised from pp45-46 of Roberts, N (1989) The Holocene: an
environmental history. Blackwell: Oxford)
> I take Don's point about ratios -- if the plant foods are cooked. The
> question now is: when did cooking become universal?
This is a very hard question to answer. I and some colleagues have spent
several years trying to devise methods for detecting bone that has been
cooked. All the chemical signs of cooking that we can find also occur at
a much slower rate in the decay processes which occur after burial.
(e.g. Roberts SJ, Smith CI, Millard A & Collins M 2002 The taphonomy of
cooked bone: characterising boiling and its physico-chemical effects,
Archaeometry 44(3) 485-494.) One can find and identify burnt materials
relatively easily, and although burning is not cooking, this is about the
best we can do.
> Homo erectus began to appreciate the value of fire around 350,000 years ago.
I think this must be a reference to the deposits at Zhoukoudien (the
"Peking Man" site), which have now been shown to be waterlain deposits
rather than ashes (Weiner S, Xu QQ, Goldberg P, Liu JY, Bar-Yosef O
Evidence for the use of fire at Zhoukoudian, China Science 281 251-253).
If there is any other evidence for ash quite this early I'd like to hear
about it.
> Although hearths have been discovered that are 100,000 years old, these are
> very rare. European Neanderthal coprolites from around 50,000 years ago,
> before their use of fire, contain no plant material. (Bryant V M,
> Williams-Dean G. The Coprolites of Man. Scientific American, January 1975.)
But ash deposits do form the major consituents of many cave sediments, e.g
Kebara, Hayonim and Tabun in Israel, with dates going back over 200,000
years, indicate prolonged use of fire but without any surviving hearth
structures. This use of fire is not unequivocally associated with a
particular hominid species, but appears to predate the speciation that led
to modern humans, and it is therefore within an appropriate timescale for
evolution of a gut dependent on cooked foods. Neanderthals are unlikely
to have contributed significantly to the modern human gene-pool, so the
contents of their coprolites are less relevant than they might othewise
be.
> It seems that it was not until Cro-Magnon's colonisation of Europe, some
> 35,000 years ago, that hearths became universal. Even then the evidence
> suggests that they were not used for cooking plants but merely for warmth.
> This is really too recent in our history for any big change in genetic
> makeup, surely?
It seems to me that the actions of Cro-Magnon people may not be indicative
of the majority of our ancestry. They were living in a marginal
environment and genetic studies of modern European mitochrondrial DNS
suggest that the majority of ancestors of modern Europeans entered Europe
after the LGM - presumably from areas where the technology and
hearth-usage were different.
Andrew
=========================================================================
Dr. Andrew Millard [log in to unmask]
Department of Archaeology, University of Durham, Tel: +44 191 334 1147
South Road, Durham. DH1 3LE. United Kingdom. Fax: +44 191 334 1101
http://www.dur.ac.uk/a.r.millard/
=========================================================================
|
