This article explores the hypothesis that key human adaptations evolved in response to environmental instability. This idea was developed during research conducted by the Smithsonian’s Human Origins Program. Natural selection was not always a matter of ‘survival of the fittest’ but also survival of those most adaptable to changing surroundings.
(Illustrations for this article coming soon.)
Paleoanthropologists – scientists who study human evolution – have developed a variety of ideas concerning how environmental conditions may have stimulated long-term human evolutionary change. Human evolution has involved the emergence of a diverse suite of species and an accumulation of adaptations, including bipedal walking, the capacity to make and depend on tools, brain enlargement, changes in growth, and the emergence of complex mental and social behavior.
An important feature of Earth history during the period of human evolution is environmental change, including cooling, drying, and wider climate fluctuations over time. The question arises, then, how environmental change may have shaped the evolution of new adaptations and the origin or extinction of early hominin species and the emergence of our species, Homo sapiens. (The word ‘hominin’ here is used to refer to branches (= lineages or species) of the human evolutionary tree since the divergence of human ancestors and chimpanzee ancestors from a common ancestor, estimated to have lived between 6 and 8 million years ago.)
How do we know that earth’s climate has changed? How do we know how quickly and how much climate has changed? One of the ways we can look at environmental change is through the record of oxygen isotopes through time.
This record of δ18O, or oxygen stable isotopes, comes from measuring oxygen in the microscopic skeletons of foraminifera (forams, for short) that lived on the sea floor. This measure can be used as an indicator of changing temperature and glacial ice over time. There are two main trends: one toward an overall decrease in temperature, the other toward a larger degree of climate fluctuation. The amount of variability in environmental conditions became greater in later stages of human evolution than in the earlier stages.
There are two isotopes of oxygen, 18O and 16O. Isotopes are forms of the same chemical element that have different atomic weights resulting from different numbers of neutrons in the nucleus. During evaporation, more 16O evaporates than 18O. Microorganisms in the ocean, such as foraminifera, are more likely to incorporate 18O than 16O in their skeletons. The ratio of the two isotopes in the skeletons of marine organisms changes as temperature changes. When the temperature drops, even more 18O is found in the forams. During glacial periods, more 16O ends up in locked away in ice. To get information about the ratio of oxygen isotopes, ships drill deep into the ocean floor to get a deep sea core that contains the remains of the forams, which can be tested to determine their isotopic composition.
Organisms and Environmental Change
All organisms encounter some amount of environmental change. Some changes occur over a short time, or might be cyclical, such as daily or seasonal variations in the amount of temperature, light and precipitation. On longer time scales, hominins experienced large-scale and unpredictable shifts in temperature and precipitation that in turn caused vast changes in vegetation – from grasslands to forests, and from extremely cold to warm climates. Hominin environments were also altered by tectonics, such as the uplift of the Tibetan Plateau, which changed rainfall patterns in northern China and altered the topography of a wide region. Tectonic activity can change the location and size of lakes and rivers. Volcanic eruptions and forest fires also altered the availability of food, water,...