Ancient Hominins Were Found in Diverse Habitats

Ancient hominin remains have been found in a variety of different habitats. While some hominins, such as Orrorin tugenensis and Ardipithecus ramidus have been found in wooded habitats, others such as Sahelanthropus tchadensis were found associated with diverse types of vegetation within a small geographic area. Reconstructions of the ancient habitat of Ardipithecus ramidus at two different Ethiopian sites suggest that this species occupied both wooded areas (the Aramis site) and wooded grasslands in which grazing animals predominated (the Gona site). Australopithecus anamensis has been found at Kanapoi and Allia Bay, Kenya, in association with another type of mosaic – an open savanna with low trees and shrubs, but with both grasslands and gallery forests nearby.

At Kanapoi, research by Dr. Jonathan Wynn on paleosols and pedogenic carbonates demonstrates the presence of these varied habitats at the time when Australopithecus anamensis inhabited the area. Other members of Au. anamensis at Allia Bay encountered a different environment. The fossil animals represent several different habitats including open floodplains, gallery forests, and dry bushlands. Isotopic studies done by Dr. Margaret Schoeninger and her colleagues indicate that most of the Allia Bay vegetation consisted of woody plants such as trees and shrubs (known as C3 vegetation).  Australopithecus anamensis at Allia Bay was thus associated with a mosaic environment, including woodlands near the ancestral Omo River and open savanna further away.

Two different types of environment – dense woodlands and open bushland – occurred in the same areas of East Africa during the period of human evolution. Climate fluctuation altered the proportion of these habitats, and thus led to repeated changes only in population density and variable conditions of natural selection.

Two Legs, Long Arms; Moving Around in Diverse Habitats

By about 4 million years ago, the genus Australopithecus had evolved a skeletal form that enabled adjustment to changes in moisture and vegetation. The best current example of adaptability in Australopithecus is apparent in the skeleton known as Lucy, which represents Au. afarensis. Lucy’s 3.18-million-year-old skeleton has a humanlike hip bone and knee joints coupled with long apelike arms, longer grasping fingers than in humans, and flexible feet for walking or climbing. This combination of features, which appears to have characterized Australopithecus for nearly 2 million years and possibly older hominins, afforded an ability to move around in diverse habitats by changing the degree of reliance on terrestrial walking and arboreal climbing. This flexibility may also have characterized earlier hominins such as Ardipithecus ramidus.

Stone Toolmaking: Gaining Access to Diverse Foods

The first known stone tools date to around 2.6 million years ago. Making and using stone tools also conferred versatility in how hominin toolmakers interacted with and adjusted to their surroundings.

Simple toolmaking by stone-on-stone fracturing of rock conferred a selective advantage in that these hominin toolmakers possessed sharp flakes for cutting and hammerstones that were useful in pounding and crushing foods. Basic stone tools thus greatly enhanced the functions of teeth in a way that allowed access to an enormous variety of foods. These foods included meat from large animals, which was sliced from carcasses using sharp edges of flakes. Bones were broken open using stones to access the marrow inside. Other tools could be used to grind plants or to sharpen sticks to dig for tubers. Tool use would have made it easier for hominins to obtain food from a variety of different sources. Tool use would have widened the diet of hominins. Meat, in particular, is a food that was obtainable in equivalent ways, with similar nutritional value, in virtually any type of habitat that early humans encountered.

Although making simple toolmaking may have developed originally in one type of environment, the carrying of stone tools over considerable distances – and becoming reliant on stone technology – may have arisen due to the benefits of altering the diet as environments changed. The oldest known stone technology – called Oldowan toolmaking – involved carrying rock over several kilometers and is found associated with a variety of ancient habitats. Redistributing stone and other resources, such as parts of animal carcasses, by transporting them may have helped hominins cope with variable habitats.

The Expanding World of Early Homo

As predicted by the variability selection hypothesis, hominins were not found solely in one kind of habitat, but rather in a variety. A major signal of the ability to tolerate different environments was the dispersal of the genus early Homo beyond Africa into Asian environments. After 1.9 million years ago, the genus Homo is found in a variety of locations in Asia, including some that are relatively far north.

Early evidence of the diversity of Homo erectus environments in Asia includes the following sites: 

• Dmanisi, Republic of Georgia, 1.85 to 1.78 Million years ago. This site has grasslands surrounded by mountains with forests. Hominins had access to lava as a raw material for tools.
• Yuanmou, China, 1.7 Million years ago. This site, located near an ancient lake, had a mixture of habitats with grasslands, bushlands and forests.
• Nihewan Basin, China, 1.66 Million years ago. The Nihewan sites were also near a lake. Hominin toolmakers experienced many changes in vegetation over time, with habitats ranging from forests to grasslands. This region may have been much more arid than others, and temperatures changed seasonally between warm and cold.
• Java, 1.66 Ma: Hominins here encountered grasslands, rivers and marine coastal environments in a tropical latitude setting.

In these locations, hominin groups encountered distinctly different environments, different plants and animals and foods, and different climatic conditions – a very wide range of temperature and strong variations in aridity and monsoonal rains.

Hominins Persisted Through Environmental Change

Environmental instability may have been a factor not only in shaping adaptations but also in contributing to the extinction of some lineages. Environmental variability associated with the extinction of large mammal species has been proposed for the southern Kenya region. Sediments, stone artifacts, and animal faunal at the site of Olorgesailie span most of the past 1.2 million years. Numerous environmental shifts are recorded in the Olorgesailie deposits. The ancient lake level and its chemistry, for example, changed frequently, and sometimes the lake dried up, leaving small wetlands and streams as the main source of water in the basin. Volcanic eruptions also blanketed the landscape in ash, killing off grass and reshaping the properties of the ecosystem.

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