Re: the role of savannas in human evolution

On Feb 10, 2:33 pm, Marc Verhaegen <m_verhae...@xxxxxxxxx> wrote:

More spiritual imagination.

Here is scientific evidence:

http://www.mc.maricopa.edu/dept/d10/asb/anthro2003/origins/hominid_journey/pottsclimate.html

The following is taken from Rick Potts' Humanity's Descent:

"Evidence from the polar regions indicates that something very
important happened to the global environment of this period. Antarctic
ice cores reveal a major expansion in the southern ice cap around 2.4
to 2.5 million years ago. The first major ice rafts are also recorded
in the Northern Hemisphere at this time. The foram thermometer,
mirroring the oxygen isotope signal of global ice, shows a critical
drop at about 2.5 million years, a brief time after a reversal in
Earth's magnetic field, known as the Gauss/Matuyama boundary. The
abrupt shift in the ice curve resulted from an increase in the heavier
form of oxygen, 18o, as massive quantities of the lighter 16o were
stolen from the oceans. This occurred at the same time that vast ice
sheets began to grow. On a global scale, Earth experienced a stepwise
cooling, a dramatic prelude to a future frigid age.

This distant warning reverberated over the continents. About 2.4
million years ago, an extensive series of windblown deposits, called
loess, began to form over much of China. Loess deposits meant that
winds were able to carry particles of sand and silt over open expanses
of low-lying vegetation. Great depths of windswept dust appeared for
the first time in central Europe and western Asia. In the Andean
highlands of South America, studies by Henry Hooghiemstra of the
University of Amsterdam record a formidable shift in fossilized
pollen, signaling a grassland expansion at the expense of forest. The
interval 2.4 to 2.5 million years ago is the marker for all of these
events. The tandem of cooling and drying was felt far and wide.

In a very real sense, this worldwide episode was a single distillation
of the kind of environmental change commonly thought to have spurred
critical developments in human evolution. The shift to open savanna
seems to have occurred in one major upheaval, so the savanna
hypothesis may be salvaged after all. Perhaps the expansion of savanna
around 2.5 million years ago was the crucial nudge that set the human
lineage on its course. An influential hypothesis advanced by Elisabeth
Vrba of Yale University takes its cue from this important event.
Vrba's turnover-pulse hypothesis has stimulated a tremendous amount of
research, spanning the disciplines of geology, paleontology,
climatology, and evolutionary biology. A turnover pulse refers to any
synchronized set of species extinctions and origins in many groups of
animals over a limited period of time. Such bursts of biological
change, according to this hypothesis, are initiated by shifts in
global climate. Climatic events not only alter the conditions of life,
they cause habitats to become more fragmented. As a consequence,
animal populations also become more divided, and new species emerge as
separated populations respond locally to their changing habitats.
Vrba's hypothesis is that the origins of new species and new adaptive
capabilities are ultimately linked to a major directional shift in
global climate. ;

Vrba states that the change in the environmental settings of early
hominids around 2.5 million years ago was exactly the kind expected by
her hypothesis. Worldwide cooling and drying were sharply mirrored in
African settings. Arid and open habitats infected eastern and southern
Africa. Woodlands and forests ebbed. The Cenozoic decline was writ
large in one wrenching leap, and as the wave of grass moved over the
African landscape, natural selection favored creatures who could
thrive in it.
Africa was an epicenter of a changing globe, and a significant measure
of it was the fossil antelopes. New species of the wildebeests and
gazelles, the dominant grazers of the African plains, made their
appearance in the fossil record between 2.5 and 2 million years ago.
According to Vrba, the antelopes of eastern and southern Africa
underwent a major episode of extinction and new species origins at
this time.
Change was not confined to the antelopes. In the Omo basin of
Ethiopia, about 2.4 million years ago, rodents from dry grasslands
replaced others typical of wetter, forested environments. As already
noted, several groups of large mammals evolved higher-crowned teeth
and longer limbs, indicative of an open habitat in the late Pliocene.
These changes are also said to correspond to an extension of grassland
around 2.4 million years ago. Shifts in fossil pollen in the Omo-
Turkana region and in the highlands of Ethiopia also hover around this
date. The challenge of a widening savanna left its mark.
Vrba suggests that our own intimate phylogeny succumbed to this same
directional change. As wooded habitats fragmented and dissolved into
open vegetation, bipeds spawned new and independent examples of the
hominid condition. Behaviors crucial to the human enterprise evolved
with the spread of a cooler, drier habitat. Our own genus, Homo, was a
founding member of the new savanna biota. Stone toolmaking and the
dental machinery of the robust australopiths evolved as adaptations
oriented to the resources of the drying, opening landscape,
evolutionary events that centered around the global climatic change
2.4 to 2.5 million years ago.
The turnover-pulse idea proposes, first, that global temperature fell
precipitously. Second, that this event caused the spread of arid
grasslands within the savanna patchwork of Africa. And, third, that
the growth of these grasslands prompted synchronized change in
hominids and other animal populations. The regional division of
populations led new species to arise, while the force of natural
selection caused new adaptations to evolve. The hominids were
converted to live in open terrain.

Vrba's proposal leans heavily on the savanna hypothesis: Directional
change to an open habitat caused profound alterations in human origin.
The idea of a turnover pulse is appealing for other reasons. It
appears to account for the era's dynamism. Evolutionary events between
3 and 2 million years ago, according to Vrba, were focused on a single
climatic alteration, and included all the events described earlier in
this chapter the formation of new species, parallelisms across many
lineages, and vital developments in the ecological history of our own
lineage. The concept of a turnover pulse during the late Pliocene has,
moreover received resounding support from climatologists, who have
discovered ir local and global archives evidence of significant change
about 2.5 million years ago, just after the Gauss/Matuyama magnetic
boundary was imprinted in sediments around the world. The
concentration of climatic and biotic change at this special point in
time is the core of this hypothesis.
Vrba has done a remarkable job of scrutinizing her own hypothesis, but
three critical questions test its relevance to human origin: What was
the nature of this climatic episode 2.5 million years before the
present? Were global change, local shifts in habitat, and developments
in hominids precisely correlated? Does directional change provide a
stronger explanation of hominid evolution during this period than
environmental fluctuation? Let us put the turnover-pulse hypothesis
and the thesis we have begun to develop here to these tests.

A curious pattern can be seen in our investigationwhenever Cenozoic
environments became cooler or drier, they manifested greater
instability at about the same time, or immediately before the main
shift. This happened with the cooling event of 2.5 million years ago.
In a paper widely cited by advocates of the turnover-pulse idea,
Nicholas Shackleton and his colleagues showed that large icebergs
formed in the northern oceans for the first time about 2.4 million
years ago. Because of recent redating of the Gauss/Matuyama magnetic
reversal, the time of this change was probably closer to 2.5 million
years ago. Before this date, the ice system of the planet was largely
confined to Antarctica, and this single ice pole controlled the heat
gradient between the equator and the poles. After 2.5 million years
ago, however, the North Pole also developed large ice sheets and began
to play a major role in governing Earth's climate.

Overshadowed by this dip in the oxygen isotope curve is a series of
intense oscillations. According to Shackleton's paper, global climate
varied a great deal before 2.5 million years ago, affecting the signal
of ice fluctuation in the North Atlantic. Indeed, a wider range of
fluctuation in the isotope curve began at least 300,000 years before
the critical date. With the approach of the 2.5-million-year mark, the
range of variation rose even more.
The curve below, from Shackleton and colleagues' original 1984 paper,
shows that the dip at 2.4 to 2.5 million was not a permanent change,
but part of a longer period of increased instability. On occasion, the
isotope readings rebounded to levels that existed prior to the dip.
Isotope measures in other ocean cores portray a slightly different
trend and pattern of oscillation. According to a Pacific Ocean core
published in I98I, climatic variation began to widen as early as 3. 1
million years ago. The widened pattern of oscillation occurred up to
2.1 I million years ago, when even larger variations began to occur.

In 1984, geologists R. Stein and M. Sarnthein examined several other
ocean cores drawn from the Atlantic Ocean, and inferred that
deviations in the climatic curve increased in both frequency and
intensity between 3.5 and I.9 million years ago. They concluded that
much of this fluctuation occurred 500,000 years on either side of the
2.5-million-year markthat is, the entire span between 3 and 2 million
years before the present.
The volume of terrestrial dust blown into the ocean is another
indication of continental drying. Measuring sediments buried off the
west coast of Africa, William Ruddiman and his colleagues found that
the amount of terrestrial dust in the ocean increased around 2.5
million years ago, supporting the idea of increased African aridity at
that time. However, many large-scale oscillations occurred in the
quantity of dust that fell to the ocean floor. Ruddiman attributes the
large, repeated shifts in the dust curve after 4 million years ago to
"an increase in the amplitude of arid/humid cycles" over geologic
time, with a major increase in fluctuation, including greater aridity,
at about 2.5 million years ago.
Further clues come from a long terrestrial record often cited by the
turnover-pulse proponents. Henry Hooghiemstra's record of fossil
pollen in the high plains of Bogota, South America, displays a
significant drying at the 2.5-million-year Rubicon. Around the same
time, a c hange also occurred in the oscillation of climate and
vegetation. The pollen record indicates many small-scale variations in
temperature and flora after 3.5 million years ago. Around 2.5 million,
these variations became less frequent but of far greater magnitude.
The key event of 2.5 million years ago was actually part of a lengthy
spell of deepening fluctuation in global climate.

Besides climatic factors, volcanic and tectonic forces controlled the
habitats occupied by hominids. There is no evidence to suggest that
these irregular, more catastrophic events were unusually potent at the
2.5-million-year mark. Rather, they exaggerated the degree of habitat
change that hominids faced time and again.

In the Omo-Turkana region of East Africa, volcanic eruptions
periodically sent enormous clouds of ash and debris over the
landscape, 4.2 and 1.4 million years ago. At least nine such events
blanketed an area of more than ten thousand square miles. In some
cases, erupted ash was carried eight hundred miles away to the Gulf of
Aden. Even the smaller eruptions repeatedly altered the vegetation and
soil chemistry over a wide area.
The Omo-Turkana study shows that arid savanna increased over time, but
the water budget of this vast region was controlled mainly by tectonic
activity, changing the flow of the Omo River and causing lakes to
appear and disappear over spans of 100,000 years. The forest and
woodland area surrounding these water sources was susceptible to
significant, recurrent alteration.
The environments of eastern and southern Africa became drier overall,
but the key global event, shifts in local habitat, and changes in
hominids were not precisely synchronized. A pollen study of the
OmoTurkana region by Raymonde Bonnefille, for example, points to a
shift to grassland habitats between 2.3 and 2.4 million years ago. In
this same region, changes in antelopes, pigs, and other animals took
place over a few hundred thousand years around the 2.3-million-year
mark. Because of astronomical effects on Earth's climate, a difference
of 200,000 years is a very important one, encompassing several
reversions in the climatic trend.

The first appearance of robust australopiths, stone tools, and
possibly the genus Homo precede the pollen shift in the Omo region.
The oldest known find of a robust australopith is the Black Skull,
Australopithecus aethiopicus, found in deposits several meters beneath
a volcanic tuff dated 2.52 million years ago. Independent studies of
the skull suggest that its peculiar combination of ancient and
advanced traits must have branched from the hominid family tree
independent of A. africanus, who was known in South Africa between 3
and 2.5 million years ago. Since aethiopicus has certain key
similarities with A. afarensis, the former may have evolved from the
latter. And since afarensis is unknown in the fossil record after
about 2.8 million years ago, this date may provide a minimum age for
the origin of aethiopicus The robust lineage of East Africa arose
before the abrupt savanna expansion posited by the turn-over-pulse
hypothesis.

The oldest stone tools known from the Omo-Turkana basin are dated
about 2.4 million years. A few early archeological sites in this
region and in Zaire and Malawi appear to be about 2.3 to 2.1 million
years old. Yet the oldest definite stone implements, from Hadar,
Ethiopia, are nearly 2.6 million years old. While these early
implements correlate roughly with the proposed turnover pulse, they
precede other signals of drying and grassland expansion in East Africa
by at least 200,000 years.

New dates on a fossil fragment discovered in 1967 near Lake Baringo,
Kenya, suggest that the oldest known member of the genus Homo may be
2.4 million years old. It has been argued that this appearance
coincides approximately with the oldest stone tools and is related to
the dramatic spread of open savanna. Other researchers have difficulty
accepting the Baringo fragment's age and identity, and see definite
signs of Homo no older than about 2 million years. But even if Homo
and flaked tools first appeared around the 2.5-million-year date, they
lie within the period of intensified oscillations that began prior to
the abrupt cooling event. Was it habitat oscillation or a single
megashift to cooler, drier habitat that figured more importantly in
the turnover of species and adaptive innovation among the hominids?
Five distinct species of early human may have inhabited the OmoTurkana
region between 2.6 and 1.8 million years ago. The turnover - pulse
idea has remarkable appeal partly because it sees a dynamism in the
hominids similar to that in other lineages of mammals. According to
Vrba's hypothesis, the spread of savanna caused a phenomenon known as
"environmental forcing"the division and extinction of lineages under
pressures associated with a particular climatic trend.

It seems to me that vacillationreversals in the trendcreated a much
more appropriate setting for this mutable era of human evolution.
This, too, is a kind of "environmental forcing." But the critical
factor was the increasing degree of habitat fluctuation, not a single
directional change. Hominids and other organisms had to survive the
spread of grassland beyond its previous limits; but our grasp of this
vital period in human origin is not complete without also asking how
these ancestors fared in subsequent moist and warm phases. The
survival conditions of human forebears cannot be rendered by any one
portrait, or any single type of biome. The change in climate at 2.5
million years ago may have been dramatic, but the longer interval of
heightened instability between 2 and 3 million years ago also cast its
influence on hominid evolution.
Robert Foley of Cambridge University has underlined the importance of
habitat fluctuation in the turnover of late Pliocene species. In his
scenario, cycles of change caused habitats to break up into smaller,
more distant fragments. Populations became disjoined and isolated for
varying lengths of time, adapting to different local conditions.
Periodic dividing and coalescing of habitats seem more essential to a
continental I radiation of species than any unidirectional habitat
trend. Environmental variabilitythe intensity and rate of
fluctuationmay well have held sway over the persistence, splitting,
changing, and extinction of of lineages.
Two proponents of the savanna-pulse idea, George Denton and Michael
Prentice, shed a different light on the timing of the climatic
decline. According to their data, the main climatic event in the late
Pliocene was 2.4 million years ago, during the buildup of ice at the
North Pole. Again, this marker seems too late to have affected the
earliest appearance of robust hominids and stone tools. Denton and
Prentice suggest, however, that climatic deterioration began around
2.8 million years ago and culminated some 400,000 years later. Their
broader "pulse" model is based on oxygen-isotope data and sea-level
change. In recent papers, Vrba has embraced this revised time scale,
but she still stresses the power of the linear trend, the rise of
savanna, as the critical source of change in hominid evolution.

The environmental code of the era was a sequence of dots and dashes.
Over this lengthy interval, numerous stops, starts, and reversals
occurred in ice buildup, ocean depth, and planetary temperature. It
was literally a tide with ebbs and flows. As water was taken up and
released by polar ice, sea level fluctuated at least fifty and up to
one hundred meters, constricting and reexposing huge areas of the
continental shelf. The amount of moisture monsoon winds carried to the
continental interior varied with distance from the ocean. Swings in
sea level, which took place over tens of thousands of years, altered
the atmospheric moisture blown into the continental basins occupied by
hominids, affecting the rainfall and vegetation of these areas.
Prentice and Denton interpret the change in sea level between 2.8 and
2.4 million years ago "as primarily reflecting Antarctic Ice ***
fluctuations with a minor but increasing component attributable to
Arctic ice caps." It is reasonable, then, to suggest that fluctuations
regulated the pattern of environmental change. Cooling and drying
occurred as part of a widening spectrum of oscillation over at least
several hundred thousand years.

How do toolmaking and other curious innovations in hominid behavior
tie in with this picture of an unsteady environment? In trying to
decide why early attempts at flaking and carrying stone persisted in
the human repertoire, we are stuck with our old conflict. Were these
behaviors adaptations to the open savanna? Or did they mainly provide
certain hominids with a way of dealing with environmental variability?
Let us consider what these novel behaviors were good for.
It has occurred to many of us who are curious about the oldest stone
tools that the hominids who processed their food partially outside of
their bodies were the most liberated of all bipeds from the demands of
any single type of environment. The new dental opportunities made
possible by stone tools meant that the toolmakers could transcend the
status quo of any single habitat or slice of time.
In any particular environment, there is a recognizable pattern to how
foods are distributed, and certain foods occur more abundantly than
others. Therefore "optimal" foraging routes and "best" food sources
can be defined in any given habitat. But even in the brevity of an
annual season, the savanna offered a changing buffet. The ultimate
test of the toolmaker way of life came as environmental extremes were
felt over longer spans of time. I believe that lithic toolmaking
persisted as a useful strategy precisely because it enabled a hominid
to switch to different resources when the old ones were gone. By
chipping rocks, certain hominids discovered a new form of versatility.
A heavy stone i id a sharp-edged flake meant that a tremendous variety
of items could be opened, cut, or crushed. Changes in food supply were
handled by making implements capable of processing whatever kinds of
food happened to be available.
For all their simplicity, fractured rocks offered a kind of buffer
against natural shifts in the resources affecting survival and
reproduction. A chipped stone first became valuable when it performed
some specific taskcutting a tough plant stem, sharpening a stick, or
slicing an animal hide. I believe that stone flaking endured not
because it encouraged this specific task, or because the original
environment of toolmaking continued to influence the hominids, but
because cutting edges and pounding stones allowed potential
differences between an arid grassland and a moist woodland to be
reconciled. Stone flaking afforded a resilient means of obtaining
needed resources in the full range of environments.
Similarly, carrying food and stones to the same sites buffered
potential conflicts between one environmental state and another. The
first time lithic tools and animal bones were brought together took
place in some specific environmental setting. But the act of
transporting items persisted and developed because of the benefits it
provided in a changing environment. Once there was transport, the fact
that stones and a particular food resource changed from being fifty
feet apart to being two miles apart did not prevent the toolmakers
from bringing these two critical resources together. Transport, much
like stone toolmaking itself, enabled hominids to survive unexpected
changes in the distribution and abundance of natural resources.

Although the earliest Oldowan flaking is more than 2.5 million years
old, the deeper implications of this primal technology may not have
been discovered until 2.2 to 1.8 million years ago. Sites in this time
range preserve the oldest evidence of wide stone transport and tool
cut marks on animal bones. Deep-sea cores, drilled from the North
Atlantic and the tropical Pacific, reveal two unusual times of oxygen-
isotope fluctuation between 2.2 and 2 million years ago. Two ocean
cores from the western Atlantic indicate an intense oscillation at
about 2.2 million years ago, and another just before 2 million years
ago. On a global scale, relatively wide climatic oscillations
coincided with the advent of distant transport and dietary changes in
human toolmakers.
On land, this period is less well known. The best environmental record
we have from this era, from the Olduvai Gorge, is 1.8 to 1.7 million
years old. At the outset of this interval, Olduvai was dominated by
moist woodland vegetation and high lake stands, an environmental state
that was maintained for about 40,000 years. Within the next 10,000
years, extremely arid conditions developed. Lake level dropped
significantly; fossil animals indicate semi-desert conditions; the
pollen record shows a shift from river forests to steppe. Shortly
before 1.75 million years ago, lake levels were again on the rise, and
closed vegetation and humid climate prevailed, until the trend was
reversed by another abrupt shift to aridity, which piled up windblown
dust about 1.7 million years ago.
The vegetation was drier and more open at the end of this sequence
than it was at the outset, but the trend was not smooth. Aridity was
interrupted by moist periods; cooling, by warming. In the shadowed
passages of time preserved in the gorge, we see that toolmakers
practiced their craft over the entire range of habitats. Long-term and
gradual shifts in rainfall and temperature, occasional deluges of
volcanic ash, tilts in the landscape from rare and awesome fracturing
of

Earth's crust together must have caused impressive rearrangements in
the resources of survival. Yet there is hardly a stratum in the most
ancient depths of Olduvai Gorge where the imprint of stone toolmakers
is absent.
To accept the anthropological dogma that the toolmakers flourished in
drier, open habitats is to imply that human adaptation was molded
primarily during periods of aridity. Alternatively, the environments
of every time span were involved; natural selection produced relative
success in all climates, wet and densely vegetated spans as well as
periods of dry grassland. Knapping and stone gathering cut across the
fluctuarions. The hominids were geared toward surviving the entire
environmental panoply.
Although it invited the attention of other meat eaters, the inclusion
of large animals in the diet also succeeded as a buffer to change. Any
major climatic shift or geologic event would have disturbed the
vegetation, altering the abundance and location of plant foods. Herds
of large animals would also have been affected, but animals do not
manifest anything like the dramatic variation in nutritional quality
and toxins associated with different kinds of plants. To eat an open
plain's zebra is much like eating a woodland's kudu. To digest an
underground tuber is not the same as eating the soft pulp of a fruit.
Eating large animals helped stabilize the diet when climate and
vegetation changed.

To select from the mosaic's cornucopia, it was necessary to follow the
dilation and contraction of edible items. Some foods were given up and
others adopted as the occasion arose. Bringing resources together
required the toolmakers to maintain a good mental map of their milieu,
and also the capacity to change the template. The toolmakers had to
respond to the opportunities of moister terrains as much as they did
to growing aridity.
The search for mobile resources, cunning links between rocks and food,
carrying things over a distance, avoidance of predatory
carnivorescomprised the package of the earliest stonesmiths. The
combination was both dynamic and critical to the future of their
descendants. Each of these novelties became especially significant in
the next round of human originthe rise of a large-bodied, large-
brained, sweaty, long-distance nomad, who would later become the sole
survivor, the stem from which all future humanity would emerge."
.