Anthropology - Paleo

This article is in the magazine Science 307:222
Balco, Rovey and Stone, 2005

we have been discussing stone tool hostinghostinghostingociation with the time
frame of 2.59 million years ago in gona. The question is
whether the culture could have moved from elsewhere.

This paper depicts the onset of the first major iceage in
north america around 2.6 to 2.53 mya (FGMNA). The onset of
cooler temperatures may have pushed tropical species from
other spots in africa towards the equator at this time,
therefore the ubrupt onset of gona tool industries is also
possibly explained as a movement from unsampled sites
elsewhere into the region. The equitorial divide might have
restricted homo to one side of the equator before FGMNAs onset
and allowed the distribution on either side there after with
some equitorial adaptation. This initial split may have led to
the developement of ergastor and erectus later.


--
Philip
- - - - - - - - - - - - - - - - - - - - - - - - - - -
Mol. Anth. Group
Mol. Evol. Hominids
Evol. of Xchrom.

Pal. Anth. Group
Sci. Arch. Aux


DNApaleoAnth at Att dot net

Philip Deitiker <> wrote:

> This article is in the magazine Science 307:222
> Balco, Rovey and Stone, 2005
>
> we have been discussing stone tool hostinghostinghostingociation with the time
> frame of 2.59 million years ago in gona. The question is
> whether the culture could have moved from elsewhere.
>
> This paper depicts the onset of the first major iceage in
> north america around 2.6 to 2.53 mya (FGMNA). The onset of
> cooler temperatures may have pushed tropical species from
> other spots in africa towards the equator at this time,
> therefore the ubrupt onset of gona tool industries is also
> possibly explained as a movement from unsampled sites
> elsewhere into the region. The equitorial divide might have
> restricted homo to one side of the equator before FGMNAs onset
> and allowed the distribution on either side there after with
> some equitorial adaptation. This initial split may have led to
> the developement of ergastor and erectus later.

So the fist glacial maximum provided a one-two punch?
--
John S. Wilkins AA#2207
web: blog: evolvethought.blogspot.com

God cheats

In sci.anthropology.paleo, John Wilkins created a
message ID news: :

> So the fist glacial maximum provided a one-two punch?

And coughed up and "r".



--
Philip
- - - - - - - - - - - - - - - - - - - - - - - - - - -
Mol. Anth. Group
Mol. Evol. Hominids
Evol. of Xchrom.

Pal. Anth. Group
Sci. Arch. Aux


DNApaleoAnth at Att dot net

Philip Deitiker wrote:
> This article is in the magazine Science 307:222
> Balco, Rovey and Stone, 2005
<snip>
> This paper depicts the onset of
> the first major iceage in north america
> around 2.6 to 2.53 mya
> (FGMNA).
<snip>

"FGMNA", of course, means
"First Glacial Maximum in North America",
as in:

G. Balco, C.W. Rovey II, J.O.H. Stone
The First Glacial Maximum in North America
Science, Vol 307, Issue 5707, 222, 14 January 2005

But it seems that the date on that "Atlanta" glacial
deposit is somewhat younger than reported above, i.e.
2.4 Ma, and possibly as old as 2.45 Ma.
Using a different calibration standard would have
given an age of 2.64 Ma.
The marine record suggests a cold phase at about
2.58 Ma.

One wonders where "around 2.6 to 2.53 mya" comes from.

Philip, would you be so kind as to
quote the relevant phostinghostinghostingage?

Thank you.

Daryl Krupa

P.S.: The beginning of the Quaternary has already
been considered to be at about 2.6 MA, based on
several other types of records; climate had been
coolish before that, but not for extended periods:



"
5. A majority of INQUA members appear to favour a
"long" Quaternary (2.6 Ma) over a "short" Quaternary
(1.8 Ma). In essence, the preference for a "long"
Quaternary reflects perceived continuity of character
over that time. For example, around 2.6 Ma, Chinese
loess deposition becomes widespread and is substantially
different in character to the underlying Red Clay
(e.g. Ding et al. 1997. Quaternary International 40, 53).
6. Around 2.6 Ma, deep sea oxygen isotope records show
the culmination of a series of cycles of increasing
glacial intensity, also hostinghostinghostingociated with the first major
inputs of ice rafted debris to the North Atlantic.
For many this marks the beginning of the "Quaternary
ice ages".
It also marks a change from precession-dominated
to obliquity-dominated climate forcing.
"

Also:



----------------------------------------------

Abstract:

Science. 2005 Jan 14;307(5707):222.

The first glacial maximum in North America.

Balco G, Rovey CW 2nd, Stone JO.

Quaternary Research Center and Department of Earth
and Space Sciences, University of Washington,
Seattle, WA 98915, USA.


Despite marine evidence for at least hostinghosting Pliocene-
Pleistocene ice sheet advances, only the most recent
one has been accurately reconstructed from terrestrial
evidence, because there are few techniques for dating
older glacial deposits. Here we show that the cosmic
ray-produced nuclides beryllium-10 and aluminum-26 can
be used to date tills that overlie paleosols.


In the Supplementary Material, we see that the date
for the paleosol developed on that earliest "Atlanta"
till is 2.4 Ma:




Rough translation of embargoed French material
on the article:



The first movement of a North-American icecap :
The Laurentide cap
advanced for the first time on North America
approximately 2,4 million years ago,
according to the first data of terrestrial source.
The continental icecaps advanced and moved back
several times during last million years, but
the scientists include/understand really only
the most recent movements.
Those took place rather recently so that one can
employ the dating with carbon-14 to measure
the age of the rock deposits left by the icecap.
For the older movements, only the indices up to now
came from marine data.
Greg Balco and its colleagues were used for themselves
of dating to the cosmic rays in order to obtain dates
for the old drifts.
The nuclides aluminium-26 and beryllium-10 are
produced in quartz exposed to cosmic rays.
When quartz is buried, the production ceases and each
nuclide déintègre at a known speed, making it possible
to calculate since how long quartz is under the ground.
The authors fixed the date of
two layers of glacial rocks in central Missouri.
Oldest were deposited
approximately 2,4 million years ago
at the time of the oldest movement
and more in the south of the Laurentide cap.
The second layer went back to approximately
1,6 to 1,8 million years.


In the Supplementary Material, we see that the date
for the paleosol developed on that earliest "Atlanta"
till is 2.4 Ma:




Abstract at:



OR

says in
news: oups.com:

>
> Philip Deitiker wrote:
>> This article is in the magazine Science 307:222
>> Balco, Rovey and Stone, 2005
> <snip>
>> This paper depicts the onset of
>> the first major iceage in north america
>> around 2.6 to 2.53 mya
>> (FGMNA).
> <snip>
>
> "FGMNA", of course, means
> "First Glacial Maximum in North America",
> as in:
>
> G. Balco, C.W. Rovey II, J.O.H. Stone
> The First Glacial Maximum in North America
> Science, Vol 307, Issue 5707, 222, 14 January 2005
>
> But it seems that the date on that "Atlanta" glacial
> deposit is somewhat younger than reported above, i.e.
> 2.4 Ma, and possibly as old as 2.45 Ma.
> Using a different calibration standard would have
> given an age of 2.64 Ma.
> The marine record suggests a cold phase at about
> 2.58 Ma.
>
> One wonders where "around 2.6 to 2.53 mya" comes from.

Me, I was trying to digest this down based on what I observed.

P.S.: The beginning of the Quaternary has already
> been considered to be at about 2.6 MA, based on
> several other types of records; climate had been
> coolish before that, but not for extended periods:

I didn't know that, again I don't keep up to date on the
paleoclimatology past 800 kya.

So in your opinion the sudden appearance of stone tools in africa
could be an artifact of regionally biased sampling and sudden shifts
in hominid population.
How should one correct for these climactic factors when considering
the latest data hostinghostinghostingessments at Gona, and the holoconclusion that this
represents the abrupt appearance of stone tool use.

--
Philip
- - - - - - - - - - - - - - - - - - - - - - - - - - -
____Groups_____
Mol Anthro
Pal Anthro
Arch. Aux
Gliadin Sci

____Sites_____
Mol. Evol. Hominids
Evol. of Xchrom.

Philip Deitiker wrote:
> says in
> news: oups.com:
>
> >
> > Philip Deitiker wrote:
> >> This article is in the magazine Science 307:222
> >> Balco, Rovey and Stone, 2005
> > <snip>
> >> This paper depicts the onset of
> >> the first major iceage in north america
> >> around 2.6 to 2.53 mya
> >> (FGMNA).
> > <snip>
> >
> > "FGMNA", of course, means
> > "First Glacial Maximum in North America",
> > as in:
> >
> > G. Balco, C.W. Rovey II, J.O.H. Stone
> > The First Glacial Maximum in North America
> > Science, Vol 307, Issue 5707, 222, 14 January 2005
> >
> > But it seems that the date on that "Atlanta" glacial
> > deposit is somewhat younger than reported above, i.e.
> > 2.4 Ma, and possibly as old as 2.45 Ma.
> > Using a different calibration standard would have
> > given an age of 2.64 Ma.
> > The marine record suggests a cold phase at about
> > 2.58 Ma.
> >
> > One wonders where "around 2.6 to 2.53 mya" comes from.
>
> Me, I was trying to digest this down based on what I observed.
>
> P.S.: The beginning of the Quaternary has already
> > been considered to be at about 2.6 MA, based on
> > several other types of records; climate had been
> > coolish before that, but not for extended periods:
>
> I didn't know that, again I don't keep up to date on the
> paleoclimatology past 800 kya.
>
> So in your opinion the sudden appearance of stone tools in africa
> could be an artifact of regionally biased sampling and sudden shifts
> in hominid population.
> How should one correct for these climactic factors when considering
> the latest data hostinghostinghostingessments at Gona, and the holoconclusion that this

> represents the abrupt appearance of stone tool use.
>
> --
> Philip
> - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ____Groups_____
> Mol Anthro
> Pal Anthro
> Arch. Aux
> Gliadin Sci
>
> ____Sites_____
> Mol. Evol. Hominids
> Evol. of Xchrom.

"Daryl Krupa" <> says in
news: oups.com:

[Was apparently speachless]

Philip Deitiker wrote:
> says in
> news: oups.com:
>
> >
> > Philip Deitiker wrote:
> >> This article is in the magazine Science 307:222
> >> Balco, Rovey and Stone, 2005
> > <snip>
> >> This paper depicts the onset of
> >> the first major iceage in north america
> >> around 2.6 to 2.53 mya
> >> (FGMNA).
> > <snip>
<snip>
> > One wonders where "around 2.6 to 2.53 mya" comes from.
>
> Me, I was trying to digest this down based on what I observed.

Balco, Rovey and Stone, they say:
"We measured 26Al and 10Be in quartz from paleosols in the
hostinghostinghostinghostingpoorwill and Atlanta formations (table S1) and found that
the Atlanta till was deposited 2.41 ± 0.14 million years ago (Ma)."

Also:
"Given our dating uncertainty, this is likely
the same advance suggested by negative 18O excursions
in Gulf of Mexico sediments of similar age (4),
although the present uncertainties in the half-lives of
26Al and 10Be (2), as well as in the time scales for
the marine records, make it difficult to correlate
either event with the major increase in North Atlantic
ice-rafted debris near 2.5 Ma
(shown in Fig. 1 by the abrupt decrease in CaCO3 percentage) (5).
The idea that Northern Hemisphere continental ice sheets
first formed 2.7 to 2.4 Ma has previously been based on
inference from marine sediments.
The Atlanta till is direct terrestrial evidence that
the Laurentide Ice Sheet did in fact develop and advance to
its full extent during this time interval."

Their Fig. 1. (B) shows an abrupt cooling at 2.5 Ma in
the North Atlantic deep-sea core CaCO3 and 18O records.

They cite Shackleton, et al. (1) re: the core data,
who say in their abstract:
"... the first major horizon of ice-rafting occurred
at about 2.4 Myr, and was preceded by a minor pulse of
ice-rafting at about 2.5 Myr."
On p.622 they say:
"Paleomagnetic straigraphy and nannofossil stratigraphy
in this site give identical estimates for the age of
the first major northern hemisphere glacial event about
2.37 Myr."
....
"A brief ice-rafting episode, coinciding with positive
18O values, is observed at about 2.5 Myr."
They also note (on p. 623) that
"glaciation in Iceland occured as early as 3.1 Mya",
though they also note that this did not indicate European
glaciation at that time, and that
"the scale of [nannofossil] variation was much less
before 2.4 Myr than it was after the event at that time."

All indications in the Balco, et al. article are that
there was a transition to repetivie glacial-interglacial
climate cycles in the northern hemisphere starting at
about 2.4 Ma. (Or at about 2.4 Mya, if you prefer.)

Therefore,
> >> This paper depicts the onset of
> >> the first major iceage in north america
> >> around 2.6 to 2.53 mya
is not quite accurate.
More detailed chronology can be seen below.


> P.S.: The beginning of the Quaternary has already
> > been considered to be at about 2.6 MA, based on
> > several other types of records; climate had been
> > coolish before that, but not for extended periods:
>
> I didn't know that, again I don't keep up to date on
> the paleoclimatology past 800 kya.
>
> So in your opinion the sudden appearance of stone tools
> in africa could be an artifact of regionally biased
> sampling and sudden shifts in hominid population.

That's possible; actually, I have no opinion on that matter,
although I am comfortable with the thought that a major shift
in global climate would increase pressure to innovate new
technologies.
I am also comfortable with the idea that a type of fossil
may appear suddenly in the record without necessarily
indicating a sudden development; the range of a population
might extend into the site's area some time after that
population developed its identifying characteristic.
Similarly, a sudden appearance need not mean a sudden
shift in a population's range; a might date when the range
of the population expanded to include the site, without
indicating that the population had been extirpated elsewhere.
It might also mean that the site had "suddenly" become
habitable and/or exploitable by the population, i.e. the
population's range was restricted to other areas before
conditions "improved" locally.
And there is always the possibility that taphonomic
conditions were not amenable to preservation of a certain
type of fossil before a certain time.
Or even that the appropriate source of raw materials for
the particular type of tools was not available until a
certain time, due to obscuring overburden that had first to
be removed by erosion, for example.

> How should one correct for these climactic factors when
> considering the latest data hostinghostinghostingessments at Gona, and the
> holoconclusion that this represents the abrupt appearance
> of stone tool use.

You might start by checking out the results from records
from sites closer to Gona, e.g. the 1994 paper by
Tiedemann, et al. on the record of changes in airborne dust
supply to the Atlantic (from the Sahara) in the Pliocene (2).

Another 1998 paper by Hauf and Teidemann on the climatic
effects of the gradual closure of the Central American
Seaway, in _Nature_ (3), gives similar results.

Marlow, et al. had an article in _Science_ in 2000 on
sea-surface temperatures near south-west Africa (4).
You could try to look at it here:



A short popular article pointing to Marlow is here:




More on investigations of changes in the south-west
African Benguela Current:




Background article on Pliocene / Quaternary climate:

A quick background to the Pliocene



(Note the difficulty of transferring what is essentially
proxy climatic data from ocean cores to continental Africa.)


There are other, regional effects on climate to consider
too, e.g. orogeny:

The uplift of the Ruwenzori Mountains in Uganda and
its bearing on climate change in Africa and hominid evolution



Correction for climatic factors at Gona would probably
require a climatic record from Gona or nearby.
I doubt that a detailed record has yet been recovered.
The direct influence of climate change can only be inferred
until such a record is available.

Best to keep one's comments about dates and influences
of far-afield climatic change on the environment at Gona
well-qualified, with "maybes", "perhapses" and "possiblies".

Daryl Krupa


Ref's:

(1)

Nature 307, 620 - 623 (16 February 1984); doi:10.1038/307620a0

Oxygen isotope calibration of the onset of ice-rafting and
history of glaciation in the North Atlantic region

N. J. Shackleton, J. Backman, H. Zimmerman, D. V. Kent,
M. A. Hall, D. G. Roberts, D. Schnitker, J. G. Baldauf,
A. Desprairies, R. Homrighausen, P. Huddlestun,
J. B. Keene, A. J. Kaltenback, K. A. O. Krumsiek,
A. C. Morton, J. W. Murray & J. Westberg-Smith

Abstract

We report here that DSDP Site 552A, cored with the hydraulic
piston corer on the west flank of Rockall Bank, recovered an
undisturbed sequence of alternating white deep-sea carbonate
oozes and dark-coloured layers that are rich in glacial debris.
Oxygen isotope hostinghostinghostinghostingysis of the sequence together with detailed
nannofossil and palaeomagnetic stratigraphy shows that
the first major horizon of ice-rafting occurred at
about 2.4 Myr, and was preceded by a minor pulse of
ice-rafting at about 2.5 Myr.
The carbon isotope record shows that the site has been
bathed by a water mhostinghostinghosting of similar characteristics to
present-day North Atlantic deep water at least since
3.5 Myr.


(2)

PALEOCEANOGRAPHY, VOL. 9, NO. 4, PAGES 619-638, 1994

Astronomic timescale for the Pliocene Atlantic d18O and
dust flux records of Ocean Drilling Program site 659

Ralf Tiedemann
Michael Sarnthein
Nicholas J. Shackleton

Abstract

High-resolution benthic oxygen isotope and dust flux records
from Ocean Drilling Program site 659 have been hostinghostinghostinghostingyzed to
extend the astronomically calibrated isotope timescale for
the Atlantic from 2.85 Ma back to 5 Ma. Spectral hostinghostinghostinghostingysis of
the d18O records indicates that the 41-kyr period of Earth's
orbital obliquity dominates the Pliocene record.
This is shown to be true regardless of fundamental changes in
the Earth's climate during the Pliocene.
However, the cycles of Sahelian aridity fluctuations indicate
a shift in spectral character near 3 Ma.
>From the early Pliocene to 3 Ma, the periodicities were
dominantly precessional (19 and 23 kyr) and remained strong
until 1.5 Ma.
Subsequent to 3 Ma, the variance at the obliquity period
(41 kyr) increased.
The timescale tuned to precession suggests that the Pliocene
was longer than previously estimated by more than 0.5 m.y.
The tuned ages for the magnetic boundaries Gauss/Gilbert and
Top Cochiti are about 6-8% older than the ages of the
conventional timescale.
A major phase of Pliocene northern hemisphere ice growth
occurred between 3.15 Ma and 2.5 Ma.
This was marked by a gradual increase in glacial Atlantic
d18O values of 1% and an increase in amplitude variations
by up to 1.5%, much larger than in the Pacific deepwater
record (site 846).
The first maxima occurred in cold stages G6-96 between
2.7 Ma and 2.45 Ma.
Prior to 3 Ma, the isotope record is characterized by
predominantly low amplitude fluctuations (<0.7%).
When obliquity forcing was at its minimum between
4.15 and 3.6 Ma and during the Kaena interval,
d18O amplitude fluctuations were minimal.
>From 4.9 to 4.3 Ma, the d18O values
decreased by about 0.5%, reaching
a long-term minimum at 4.15 Ma,
suggesting higher deepwater
temperatures or a deglaciation.
Deepwater cooling and/or an increase
in ice volume is indicated by a series
of short-term d18O fluctuations between
3.8 and 3.6 Ma.


(3)

Nature 393, 673 - 676 (18 June 1998); doi:10.1038/31447

Effect of the formation of the Isthmus of Panama on
Atlantic Ocean thermohaline circulation

GERALD H. HAUG AND RALF TIEDEMANN

Abstract

The Late Cenozoic closure of the seaway between the North and
South American continents is thought to have caused extensive
changes in ocean circulation and Northern Hemisphere climate.
But the timing and consequences of the emergence of the Isthmus
of Panama, which closed the seaway, remain controversial.
Here we present stable-isotope and carbonate sand-fraction
records from Caribbean sediments which, when compared to
Atlantic and Pacific palaeoceanographic records, indicate that
the closure caused a marked reorganization of ocean circulation
starting 4.6 million years ago.
Shallowing of the seaway intensified the Gulf Stream and
introduced warm and saline water mhostinghostinghostinges to high northern
lahostinghostinghostingudes.
These changes strengthened deep-water formation in the Labrador
Sea over the next million years - as indicated by an increased
deep-water ventilation and carbonate preservation in the
Caribbean Sea - and favoured early Pliocene warming of the
Northern Hemisphere.
The evaporative cooling of surface waters during North Atlantic
Deep Water formation would have introduced moisture to the
Northern Hemisphere.
Although the pronounced intensification of Northern Hemisphere
glaciation between 3.1 and 2.5 million years ago substantially
lagged the full development of North Atlantic Deep Water
formation, we propose that the increased atmospheric moisture
content was a necessary precondition for ice-sheet growth,
which was then triggered by the incremental changes in the
Earth's orbital obliquity.

>From the end of the article:
"High-amplitude fluctuations in the Earth's obliquity
(low tilt angle) triggered cold summers in the Northern
Hemisphere, and prepared the way for strengthening of
the glacial-interglacial 41-kyr cycles during late
Pliocene and early Pleistocene.
However, a pronounced long-term minimum in obliquity amplitude
fluctuations occurred between 4.5 and 3.1 Myr.
The 18O records of sites 659, 846 and 999 show that during
this unfavourable orbital configuration there may have been
several failed attempts of the climate system to start
the glaciation, for example during 4.1-3.9 Myr and 3.5-3.3 Myr.
We therefore suggest that the progressive increase in obliquity
amplitudes between 3.1 and 2.5 Myr was the final trigger for
amplification and continuation of the long-term expansion of
Northern Hemisphere ice sheets after the necessary
preconditions were met 4.6-3.6 Myr ago by formation of the
Isthmus of Panama."


(4)

Science, Vol 290, Issue 5hostinghosting0, 2288-2291 , 22 December 2000
[DOI: 10.1126/science.290.5hostinghosting0.2288]

Upwelling Intensification As Part of the Pliocene-Pleistocene
Climate Transition
Jeremy R. Marlow, Carina B. Lange, Gerold Wefer, Antoni Rosell-Melé

Abstract

A deep-sea sediment core underlying the Benguela upwelling
system off southwest Africa provides a continuous time series
of sea surface temperature (SST) for the past 4.5 million years.
Our results indicate that temperatures in the region have
declined by about 10°C since 3.2 million years ago.
Records of paleoproductivity suggest that this cooling was
hostinghostinghostingociated with an increase in wind-driven upwelling tied to
a shift from relatively stable global warmth during the
mid-Pliocene to the high-amplitude glacial-interglacial cycles
of the late Quaternary.
These observations imply that Atlantic Ocean surface water
circulation was radically different during the mid-Pliocene."

>From the middle of the article:

"We have subdivided all the records into five phases to aid
interpretation.
Phase I (4.6 to 3.2 Ma, mid-Pliocene) is characterized by
warm SSTs [Sea Surface Temperatures] with three cool excursions
(1° to 2°C at 4.1, 3.7, and 3.2 Ma) that are probably related to
increased Antarctic glaciation.
The sharp cooling at 3.2 Ma marks the onset of a prolonged
gradual cooling (Phase II, 3.2 to 2.1 Ma, late Pliocene)
concomitant with the initiation of NHG [Northern Hemisphere
Glaciation], with major cooling excursions (2°C at 2.8 and 2.5 Ma)
equivalent to events in the ice volume records.
The transition from Phase II to III is marked by a rapid decline
in SST (2° to 3°C at 2.1 to 1.9 Ma) that is coincident with
the onset of more intense glacials.
Phase III (2.0 to 1.4 Ma) highlights an apparent pause in
the cooling trend but with increasing variability that possibly
reflects the increased amplitude in the ice volume record at
41-ky obliquity frequencies, although the average sampling
interval in the SST record (~hostinghosting ky) does not allow a detailed
orbital scale interpretation.
Phase IV includes a SST "crash" that is coincident with further
intensification of global ice volume during glacials, followed
by a transition at 0.6 Ma to fluctuating SSTs similar to the
~100-ky G-IG cyclicity (Phase V) and with an amplitude (5° to 7°C)
comparable to other late Quaternary U37K' reconstructions for
the BC [Benguela Current]."
....
"Previous records of aeolian dust flux to marine sediments
suggested increased trade wind strength and aridification of
Africa throughout the cooling transition.
This increase in atmospheric circulation would have been driven
by a steeper pole-equator temperature gradient owing to the
development of the bipolar cryosphere (during Phase II).
We hypothesize that the intensification of trade winds initiated
a positive feedback cycle, whereby increased trade wind-driven
upwelling enhanced the long-term (>100 ky) "leak" of CO2 from
the global ocean-atmosphere system as sedimentary organic carbon.
A consequent reduction in greenhouse forcing could have
contributed to further cooling and bipolar ice sheet expansion,
leading to further amplification of the pole-equator temperature
gradient.
The increase in aridity would have increased the supply of
nutrients to the oceans that would have been necessary to
sustain an increase in the biologically driven ocean carbon pump.
A mechanism of this type working on time scales of >100 ky may
have been an important component of changes in the global carbon
cycle and may have been partly responsible for allowing the
climatic transition to proceed (4, 10, 37-39).
Our results for the BC upwelling system suggest that the
mechanism became pronounced at 2.1 to 1.9 Ma and intensified
during the period leading to the onset of the 100-ky G-IG cycles
at ~0.6 Ma."

The intensification of BC upwelling would have had a direct regional
influence by affecting the climate of southern Africa. A reduction in
onshore precipitation owing to cooler coastal surface waters would have
compounded the shift from mesic to xeric conditions observed in this
region during the Pliocene-Pleistocene (18, 40), a transition that may
have forced local speciation events (40, 41). For example, the emerging
occurrence of the Homo genus at ~2 Ma in the region coincided with a
large increase in mammals adapted to grazing and arid conditions (41).
Both events appear to be synchronous with the abrupt cooling and
productivity increase of the BC at 2.1 to 1.9 Ma."

goddam Beta version to blame: see other reply, please

More info on African climate and its relationship to hominid
evolution:

African climate change and faunal evolution during
the Pliocene-Pleistocene
Peter B. deMenocal
Earth and Planetary Science Letters 220 (2004) pp. 3-24


Daryl Krupa