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Prof. Owen sampling sediments from a trench at
Olorgesailie, southern Kenya |
Our human ancestors roamed across
the continent of Africa for several millions of years and
evolved in response to changing environments. Evidence of their
activities is now preserved as stone tools and as fragmentary
skeletons in areas favorable for preservation. This
investigation has focused on one such location, in the southern
Kenya Rift Valley, at a place called Olorgesailie, which was
once the home of Homo erectus.
Abundant stone tools and one
pre-human skull have been found in the Olorgesailie Formation — a
series of lake, wetland, river and soil deposits that formed 1.2
million to 500,000 years ago. These ancient sediments provide a
basis for understanding environmental change during an important
period of human evolution. Several ideas have been proposed to
explain the forces that drove the development of our ancestors.
The Savanna Hypothesis, for example, suggests that our ancestors
started to walk on two legs as a response to a sustained
expansion of grasslands that opened up new habitats. In
contrast, the Variability Selection hypothesis proposes that
temporally variable conditions result in the replacement of
habitat-specific adaptations by evolutionary trends that
increased pre-human adaptability (increased intelligence, greater
social complexity) and therefore allowed the development of
flexible responses to shifting environments. This idea,
originally suggested by Professor Rick Potts (Smithsonian
Institute), implies that the driving force for human
evolution is environmental instability.
This research project has
attempted to identify just how much and how frequently
environments varied during an important phase in early human
development, which allows an assessment of the competing ideas
concerning what has driven human evolution. The work was carried
out on previously well-dated sediments that provided a time
framework for environmental reconstruction and has involved a
variety of geological studies including: field analysis of the
sediments; mineralogy and geochemical characteristics; and
especially the use of microscopic fossils, mainly single celled
algae called diatoms. These live in rivers, lakes, swamps and
damp soils with individual species being very sensitive to their
environment (water chemistry, water depth, nutrients, light
levels, and so on). Different types become common, or die out,
as environments change. Importantly, they also posses silica
skeletons that are easily preserved in sediments. Consequently,
when they die and are buried they leave behind a record of past
environments. |
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Scanning electron microscope image of Diadesmis
confervacea – a shallow water diatom |
Studies of the deposits at
Olorgesailie have shown that a complex mosaic of habitats that
changed rapidly and frequently characterized the area. These
findings contrast with other studies that have argued that there
was a stable period of expanded lakes through large parts of
eastern Africa during the time period in question. In contrast,
the southern Kenya rift experienced dry periods that alternated
with wetter phases. Fresh-water lakes and swamps formed, became
saline, and dried out only to return again. The ancestral humans
that inhabited the area 1.2 million to 500,000 years ago would
have thus experienced substantial environmental shifts that
varied with seasons, decades, and over time spans of hundreds to
thousands of years. These are conditions that argue against the
Savannah Hypothesis and for Variability Selection.
The natural forces that once
drove our evolution are still at work today. In the past,
environments changed for entirely natural reasons with climate
varying on seasonal, decadal and longer time scales. These
forces also changed the face of Africa, expanding and shrinking
deserts. For instance, lakes and rivers flowed across much of
the Sahara Desert between about 10,000–4,000 years ago. As
climates deteriorated African peoples were forced to migrate,
leading, for example, to the introduction of early pastoralism
to northwest Kenya about 4,000 years ago. Climate and
environmental change is therefore normal as is the necessity for
humans to respond physically over very long periods and
culturally on shorter time scales. However, we now face new
forces that are driven by our own activities,which have raised
greenhouse gases in the atmosphere to levels not reached for
hundreds of thousands of years. These changes have been
geologically very rapid and will likely result in major
environmental shifts. Given the evidence from Olorgesailie and
other parts of Africa, it is perhaps worth remembering that as a
species and as members of civilizations just how sensitive we
are to our environment. |
