A key goal in the study of human origins is to learn about the environmental pressures that may have shaped human evolution. As indicated in Chapter 7, scientists use a variety of techniques to reconstruct ancient environments. These include stable isotopes, core samples from oceans and lakes, windblown dust, analysis of geological formations and volcanoes, and fossils of ancient plant and animal communities. Such studies have provided valuable information about the environmental context of early Homo.
The early hominin species covered previously, such as Ardipithecus ramidus and Australopithecus afarensis, evolved during the late Pliocene epoch. The Pliocene (5.3 million to 2.6 million years ago) was marked by cooler and drier conditions, with ice caps forming permanently at the poles. Still, Earth’s climate during the Pliocene was considerably warmer and wetter than at present.
The subsequent Pleistocene epoch (2.6 million years to 11,000 years ago) ushered in major environmental change. The Pleistocene is popularly referred to as the Ice Age. Since the term “Ice Age” tends to conjure up images of glaciers and woolly mammoths, one would naturally assume that this was a period of uniformly cold climate around the globe. But this is not actually the case. Instead, climate became much more variable, cycling abruptly between warm/wet (interglacial) and cold/dry (glacial) cycles. The climate pattern was likely influenced by changes in Earth’s elliptical orbit around the sun. As is shown in Figure 10.2, each cycle averaged about 41,000 years during the early Pleistocene; the cycles then lengthened to about 100,000 years starting around 1.25 million years ago. Since mountain ranges, wind patterns, ocean currents, and volcanic activity can all influence climate pattern, climate change had extreme effects on the environment in some regions but less effects on others.
For a present-day example with which you might be familiar, consider the El Niño weather pattern. This is where warming of the Pacific Ocean in the equator region influences rainfall, hurricane frequency, and other weather activity in different parts of the world. During El Niño years, some areas get more rainfall than average and some get less. A recent El Niño in 2017 produced catastrophic flooding along the Peruvian coast, and one in 2015 led to drought and severe bushfires in Australia. If El Niños, despite being a predictable and well-known occurrence, can cause so much disruption to our technologically advanced society, imagine how vulnerable our ancestors must have been to climate change. An adaptive strategy that could buffer against this kind of uncertainty would have been extremely valuable.
Figure \(\PageIndex{1}\): Temperature estimates during the last five million years, extrapolated from deep-sea core data. Note both the lower temperatures and the increased temperature oscillations starting at 2.6 million years ago, the start of the Pleistocene epoch. Glacial/interglacial cycles during the early part of the epoch are shorter, each averaging about 41,000 years.
Data on ancient geography and climate help us understand how our ancestors moved and migrated to different parts of the world, and the constraints under which they operated. When periods of global cooling dominated, sea levels were lower as more water was captured as glacial ice. This exposed continental margins and opened pathways between land masses. During glacial periods, the large Indonesian islands of Sumatra, Java, and Borneo were connected to the Southeast Asian mainland, while New Guinea was part of the southern landmass known as greater Australia. There was a land bridge connection between Britain and continental Europe, and an icy, treeless plain known as Beringia connected Northern Asia and Alaska. At the same time, glaciation made some northern areas inaccessible to human habitation. For example, there is evidence that hominin species were in Britain 950,000 years ago, but it does not appear that Britain was continuously occupied during this period. These early humans may have died out or been forced to abandon the region during glacial periods.
In Africa, paleoclimate research has determined that grasslands (shown in Figure 10.3) expanded and shrank multiple times during this period, even as they expanded over the long term (deMenocal 2014). From studies of fossils, paleontologists have been able to reconstruct Pleistocene animal communities and to consider how they were affected by the changing climate. Among the African animal populations, the number of grazing animal species such as antelope increased. Since our early ancestors were also part of this animal community, it is informative to consider how climate change caused changes in the home ranges and migration patterns of animals. Although the African and Eurasian continents are connected by land, the Sahara desert and the mountainous topography of North Africa serve as natural barriers to crossing. But the fossil record shows that animal species moved back and forth between Africa and Eurasia during the Pliocene and Pleistocene epochs. During the early Pleistocene, there is evidence of African mammal species such as baboons, hippos, antelope, and African buffalo migrating out of Africa into Eurasia during periods when drier conditions extended out from Africa into the Middle East (Belmaker 2010).
Figure \(\PageIndex{2}\): A savanna grassland in East Africa. Habitats such as this were becoming increasingly common during the Pleistocene.
This changing environment was undoubtedly challenging for our ancestors, but it offered new opportunities for hominins to make a living. One solution adopted by some hominins was to specialize in feeding on the new types of plants growing in this landscape. As discussed in the previous chapter, the robust australopithecines probably developed their large molar teeth with thick enamel in order to exploit this particular dietary niche. Chemical analyses of robust australopith teeth show an isotopic signature of a diet where grasses and sedges are prominent, such as papyrus.
Members of the genus Homo took a different route. Faced with the unstable African climate and shifting landscape, they evolved bigger brains that enabled them to rely on cultural solutions such as crafting stone tools that opened up new foraging opportunities. This strategy of behavioral flexibility served them well during this unpredictable time and led to new innovations such as increased meat-eating, cooperative hunting, and the exploitation of new environments outside Africa.
Figure \(\PageIndex{1}\): Temperature estimates during the last five million years, extrapolated from deep-sea core data. Note both the lower temperatures and the increased temperature oscillations starting at 2.6 million years ago, the start of the Pleistocene epoch. Glacial/interglacial cycles during the early part of the epoch are shorter, each averaging about 41,000 years.
Figure \(\PageIndex{2}\): A savanna grassland in East Africa. Habitats such as this were becoming increasingly common during the Pleistocene.