9.1: Hominins, Paleoenvironment, and Evolution

DEFINING HOMININS

It is through our study of our hominin ancestors and relatives that we are exposed to a world of “might have beens”: of other paths not taken by our species, other ways of being human. But in order to better understand these different evolutionary trajectories, we must first define the terms we are using. If an imaginary line were drawn between ourselves and our closest relatives, the great apes, bipedalism (or habitually walking upright on two feet) is where that line would be. Hominin, then, means everyone on “our” side of the line: humans and all of our extinct bipedal ancestors and relatives since our divergence from the last common ancestor (LCA) with chimpanzees.

Definition: last common ancestor (LCA)

The hypothetical final ancestor (or ancestral population) of two or more taxa before their divergence.

Historic interpretations of our evolution, prior to our finding of early hominin fossils, varied. Debates in the mid-1800s regarding hominin origins focused on two key issues:

1. Where did we evolve?
2. Which traits evolved first?

Charles Darwin hypothesized that we evolved in Africa, as he was convinced that we shared greater commonality with chimpanzees and gorillas on the continent (Darwin 1871). Others, such as Ernst Haekel and Eugene Dubois, insisted that we were closer in affinity to orangutans and that we evolved in Eurasia where, until the discovery of the Taung Child in South Africa in 1924, all humanlike fossils (of Neanderthals and Homo erectus) had been found (Shipman 2002). Adding to this debate was the discovery of the Piltdown Man in England, which turned out later to be a forgery of a modified orangutan mandible and medieval human skull.

Within this conversation, naturalists and early paleoanthropologists (people who study human evolution) speculated as to which human traits came first. These included the evolution of a big brain (encephalization), the evolution of the strange way in which we move about on two legs (bipedalism), and the evolution of our strange flat faces and small teeth (indications of dietary change). Original hypotheses suggested that in order to be motivated to change diet and move about in a bipedal fashion, the large brain needed to have evolved first. And, until research picked up in Africa, fossil finds of species mentioned above predominantly had larger heads.

Definition: paleoanthropologists

 Researchers that study human evolution.

Definition: encephalization

Expansion of the brain.

We now know that bipedal locomotion is one of the first things that evolved in our lineage, with early relatives having small brains and more apelike dentition. In this chapter, we will tease out the details of what this looks like in terms of morphology (i.e. the study of the form or size and shape of things; in this case, skeletal parts).

Definition: morphology

The study of the form or size and shape of things; in this case, skeletal parts.

We also know that early human evolution occurred in a very complicated fashion. We have multiple species (multiple genera), diverse in the extent to which they move like us and the diets on which they subsist. Specimen finds have been made all along the East African Rift System (EARS; in Ethiopia, Kenya, Tanzania, and Malawi), in limestone caves in South Africa, and in Chad. Dates of these early relatives range from around 7 million years ago (mya) to around 1 mya, overlapping temporally with members of our genus, Homo (Figure \(\PageIndex{1}\)).

Definition: East African Rift System (EARS)

This term is often used to refer to the Rift Valley, expanding from Malawi to Ethiopia. This active geological structure is responsible for much of the visibility of the paleoanthropological record in East Africa.

Figure \(\PageIndex{1}\): East Africa Rift System.

Yet there is still so much to understand. Modern debates now look at the relatedness of these species to us and to one another. Discussions regarding which of these species were able to make and use tools continue. Every site discovery in the patchy hominin fossil record tells us more about our evolution. New scientific techniques provide us with insight into the diets, environments, and lifestyles of these ancient relatives that was not available to researchers even ten years ago.

Definition: site

A place in which evidence of past societies/species/activities may be observed through archaeological or paleontological practice.

A Note on Brain Size

It is worth noting that while brain size expansion is seen primarily in our genus, Homo, earlier hominin brain sizes were highly variable between and within taxa, from 300 cc (cranial capacity, cm³), estimated in Ardipithecus, to 550 cc, estimated in Paranthropus boisei. The lower estimates are well within the range of variation of nonhuman extant great apes, and body size variability also plays a role in the interpretation of whether brain size could be considered large or small for a particular species or specimen.

Increases in brain size do not necessarily correlate with an increase in intelligence in animals, especially if body size is not taken into consideration. However, the brain is an expensive tissue to build and maintain. Researchers therefore argue that the cost of maintenance must yield some evolutionary benefit. This is more easily understood in hominins where the stone tool record (an indication of behavior and intelligence) is well associated with the species.

Ancestral and Derived Traits

In Chapter 5, you were introduced to ways of organizing living taxa. In the past, taxonomy was primarily based on morphology (i.e., the physical features of organisms). Today they are tied to known relationships based on molecular phylogeny (such as based on DNA) or a combination of the two. This technique is complicated when applied to living taxa, but it becomes immensely more difficult when we seek to categorize ancestor-descendant relationships in long-extinct forms, where molecular information is no longer preserved. In many ways, we find ourselves falling back on morphological comparisons (often on teeth and partially fossilized skeletal material) in the absence of genetic material.

Definition: taxonomy

The science of grouping and classifying organisms.

Definition: phylogeny

The study of the evolutionary history of groups of organisms.

It is here that we turn to the related concepts of cladistics and phylogenetics. Cladistics groups organisms according to their last common ancestors based on shared derived traits. These are traits (in the case of early hominins, morphological) that are evolved, differing from those seen in earlier populations or forms. These new or modified traits or characteristics provide evidence of evolutionary relationships, and organisms with the same derived traits are grouped in the same clade (Figure \(\PageIndex{2}\)). For example, if we use feathers as a trait, we can group pigeons and ostriches into the clade “birds.” A good example we will see in this chapter is the grouping of what is known as the “Robust Australopithecines,” whose cranial and dental features differ from those of other hominins of a similar or earlier time period and can therefore be considered derived.

Definition: cladistics

The field of grouping organisms into those of shared ancestry.

Definition: phylogenetics

The study of relationships between organisms based on evolutionary similarities and differences.

Figure \(\PageIndex{2}\): Clades refer to groups of species or taxa that share a common ancestor.

It is also worth noting that species designations for early hominin specimens are often highly contested. This is, in part, due to the fragmentary nature of the fossil record, the large timescale (in millions of years) with which paleoanthropologists need to work, and because of the difficulty in evaluating whether morphological differences and similarities are due to meaningful phylogenetic or biological differences or subtle differences/variation in niche occupation or time. In other words, do morphological differences indicate different species? How would classifying species in the paleoanthropological record compare with classifying living species today, for whom we can sequence genomes and observe lifestyles?

There are also broader philosophical differences among researchers when it comes to paleo-species designations. Some scientists, known as “lumpers,” argue that large variability is expected among multiple populations in a given species over time. These researchers will therefore prefer to “lump” specimens of subtle differences into single taxa. Others, known as “splitters,” argue that species variability can be measured and that even subtle differences can imply differences in niche occupation that are extreme enough to mirror modern species differences. In general, splitters would consider geographic differences among populations as meaning that a species is polytypic (i.e., capable of interacting and breeding biologically but having morphological population differences). This is worth keeping in mind when learning about why species designations may be contested.

Definition: lumpers

 Researchers who prefer to lump variable specimens into a single species or taxon and who feel high levels of variation is biologically real.

Definition: splitters

Researchers who prefer to split a highly variable taxon into multiple groups or species.

Definition: polytypic

 In reference to taxonomy, having two or more group variants.

This further plays a role in evaluating ancestry. Debates over which species “gave rise” to which continue to this day. It is common to try to create “lineages” of species—determining, in other words, when one species evolves into another over time. We refer to these as chronospecies. Constructed hominin phylogenetic trees are routinely variable, changing with new specimen discoveries, new techniques for evaluating and comparing species, and, some have argued, nationalist or biased interpretations of the record. More recently, some researchers have shifted away from “treelike” models of ancestry toward more nuanced metaphors such as the “braided stream,” where some levels of interbreeding among species and populations are seen as natural processes of evolution.

Definition: chronospecies

Species that are said to evolve into another species, in a linear fashion, over time.

Finally, it is worth considering the process of fossil discovery and publication. Some fossils are easily diagnostic to a species level and allow for easy and accurate interpretation. Some, however, are more controversial. This could be because they do not easily preserve or are incomplete, making it difficult to compare and place within a specific species (e.g., the patella or knee bone). Researchers often need to make several important claims when announcing or publishing a find: a secure date (if possible), clear association with other finds, and an adequate comparison among multiple species (both extant and fossil). It is therefore not uncommon for the scientific community to know that an important find was made years before it is scientifically published.

PALIOENVIRONMENT AND HOMININ EVOLUTION

There is so much more to paleoanthropology than digging up and grouping fossil hominins: the discipline seeks to explain and understand the evolution of our ancestors’ behavior and morphology. There is no doubt that one of the major drivers (selective pressures) in hominin evolution is the environment. Large-scale changes in global and regional climate, as well as alterations to the environment, are all linked to hominin diversification, dispersal, and extinction (Maslin et al. 2014).

Environmental reconstructions often use modern analogues. Let us take, for instance, the hippopotamus. It is an animal that thrives in environments that have abundant water to keep its skin cool and moist. If the environment for some reason becomes drier, it is expected that hippopotamus populations will reduce. If a drier environment becomes wetter, it is possible that hippopotamus populations may be attracted to the new environment and thrive. Such instances have occurred multiple times in the past, and the bones of some fauna (animals, like the hippopotamus) that are sensitive to these changes give us insights into these events.

Definition: fauna

 The animals of a particular region, habitat, or geological period.

Reconstructing a paleoenvironment relies on a range of techniques, which vary depending on whether research interests focus on local changes or more global environmental changes/reconstructions. For local environments (reconstructing those of a single site or region), looking at the faunal assemblages (collections of fossils of other animals found at a site) and comparing them to animals found in certain modern environments allow us to determine if the environments in the past mirror those seen today in the region. Changes in the faunal assemblages, as well as when they occur and how they occur, tell us about past environmental changes. Other techniques are also useful in this regard. Isotopes of these fauna, for instance, tell us about the relative diets of individual fauna (e.g., using carbon isotopes to differentiate between species eating more grassland-heavy diets and those consuming bushland/tree-heavy diets) and whether the environment of individual animals was wetter or drier than the present day (e.g., nitrogen isotopes; Kingston & Harrison 2007).

Definition: paleoenvironment

An environment from a period in the Earth’s geological past.

Definition: faunal assemblages

Collections of fossils of other animals found at a site.

Global climatic changes in the distant past, which fluctuated between being colder and drier and warmer and wetter on average, would have global implications for environmental change (Figure \(\PageIndex{3}\)). These can be studied by using marine core and terrestrial soil data and by comparing these lines of evidence across multiple localities/sites/regions. These techniques allow us to use chemistry (such as nitrogen and oxygen isotopes in shells and sediments) or pollen grains (which show directly the kinds of flora surviving in an environment at a specific time period). This means that there are multiple lines of evidence that allow us to visualize global trends over millions of years (although it should be noted that the direction and extent of these changes could differ by geographic region).

Definition: flora

The plants of a particular region, habitat, or geological period.

Figure \(\PageIndex{3}\): This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, illustrates how atmospheric CO₂ has fluctuated over time and increased sharply since the Industrial Revolution.

Both local and global climatic/environmental changes have been used to understand parameters affecting our evolution (DeHeinzelin et al. 1999; Kingston 2007). There are numerous hypotheses regarding how climate has driven and continues to drive human evolution. Environmental change acts as an important keystone in hypotheses regarding the onset of several important hominin traits that are seen in early hominins and which are discussed in this chapter. Namely, the environment has been interpreted as:

1. the driving force behind the evolution of bipedalism (terrestrial locomotion on two legs),
2. the changing and diversifying of early hominin diets, and
3. the diversification of multiple early hominin species.

Here, we will explore the five most popular hypotheses.

Savannah Hypothesis or Aridity Hypothesis

The Hypothesis

This popular theory was first penned by Charles Darwin and supported by anthropologists like Raymond Dart (Darwin 1871; Dart 1925). It suggests that the expansion of the savannah (or less densely forested, drier environments) forced early hominins from an arboreal lifestyle (one living in trees) to a terrestrial one where bipedalism was a more efficient form of locomotion (Figure \(\PageIndex{4}\)). This hypothesis stems from the idea that the Last Common Ancestor (LCA) between us and chimpanzees was a knuckle-walking quadruped like chimpanzees and gorillas. However, this idea was supported by little fossil or paleoenvironmental evidence and was later refined as the Aridity Hypothesis. The Aridity Hypothesis states that the long-term aridification and, thereby, expansion of savannah biomes were drivers in diversification in early hominin evolution (deMenocal and Bloemendal 1995; deMenocal 2004). It advocates particularly for periods of accelerated aridification leading to early hominin speciation events.

Definition: Aridity Hypothesis

The hypothesis that long-term aridification and expansion of savannah biomes were drivers in diversification in early hominin evolution. 

Definition: aridification

Becoming increasingly arid or dry, as related to the climate or environment.

The Evidence

While early bipedal hominins are often associated with wetter, more closed environments (i.e., not supporting the Savannah Hypothesis), both marine and terrestrial records seem to support general cooling, drying conditions, with isotopic records indicating an increase in grasslands (i.e., colder and wetter climatic conditions) between 8 mya and 6 mya across the African continent (Cerling et al. 2011). This can be contrasted with later climatic changes derived from aeolian dust records (sediments transported to the site of interest by wind), which demonstrate increases in seasonal rainfall between 3 mya and 2.6 mya, 1.8 mya and 1.6 mya, and 1.2 mya and 0.8 mya (deMenocal and Bloemendal 1995; deMenocal 2004).

Interpretations

Despite a relatively scarce early hominin record, it is clear that two important factors occur around the time period in which we see increasing aridity. The first factor is the diversification of taxa, where high morphological variation between specimens has led to the naming of multiple hominin genera and species. The second factor is the observation that the earliest hominin fossils appear to have traits associated with bipedalism and are dated to around the drying period (as based on isotopic records). Some have argued that it is more accurately a combination of bipedalism and arboreal locomotion, which will be discussed later. However, the local environments in which these early specimens are found (as based on the faunal assemblages) do not appear to have been dry.

Figure \(\PageIndex{4}\): The African savannah grew during early hominin evolution. This may have forced early hominins from an arboreal lifestyle to a terrestrial one, where bipedalism was a more efficient form of locomotion.

Turnover Pulse Hypothesis

The Hypothesis

In 1985, paleontologist Elisabeth Vbra noticed that in periods of extreme and rapid climate change, ungulates (hoofed mammals of various kinds) that had generalized diets fared better than those with specialized diets (Vrba 1988; Vrba 1998). Specialist eaters (eating and relying primarily on specific food types) faced extinction at greater rates than their generalist (able to eat more varied and variable diets) counterparts because they were unable to adapt to new environments. This meant that specialist eater species were often confined to isolated areas with dwindling resources, whereas generalist eaters were able to move out across the landscape in search of new food sources (Vrba 2000). Thus, periods with extreme climate change would be associated with high faunal turnover: that is, the extinction of many species and the speciation, diversification, and migration of many others to occupy various niches.

Definition: ungulates

Hoofed mammals—e.g., cows and kudu.

Definition: specialist species

A specialist species can thrive only in a narrow range of environmental conditions or has a limited diet.

Definition: generalist species

A species that can thrive in a wide variety of habitats and can have a varied diet.

Definition: faunal turnover

The rate at which species go extinct and are replaced with new species.

The Evidence

The onset of the Quaternary Ice Age, between 2.5 mya and 3 mya, brought extreme global, cyclical interglacial and glacial periods (warmer, wetter periods with less ice at the poles, and colder, drier periods with more ice near the poles). Faunal evidence from the Turkana basin in East Africa indicates multiple instances of faunal turnover and extinction events, in which global climatic change resulted in changes from closed/forested to open/grassier habitats at single sites (Behrensmeyer et al. 1997; Bobe and Behrensmeyer 2004). Similarly, work in the Cape Floristic Belt of South Africa shows that extreme changes in climate play a role in extinction and migration in ungulates. While this theory was originally developed for ungulates, its proponents have argued that it can be applied to hominins as well. However, the link between climate and speciation is only vaguely understood (Faith and Behrensmeyer 2013).

Definition: Quaternary Ice Age

The most recent geological time period, which includes the Pleistocene and Holocene Epochs and which is defined by the cyclicity of increasing and decreasing ice sheets at the poles.

Definition: interglacial

A period of milder climate in between two glacial periods.

Definition: glacial

 Colder, drier periods during an ice age when there is more ice trapped at the poles.

Interpretations

While the evidence of rapid faunal turnover among ungulates during this time period appears clear, there is still some debate around its usefulness as applied to the paleoanthropological record. Specialist hominin species do appear to exist for long periods of time during this time period, yet it is also true that Homo, a generalist genus with a varied and adaptable diet, ultimately survives the majority of these fluctuations, and the specialists appear to go extinct.

Forest Hypothesis

The Hypothesis

Based on contrasting environmental evidence to the Savannah hypothesis, R. J. Rayner and colleagues (1993) hypothesized that forested environments, rather than savannahs, were a key influence on the development of bipedalism in hominins. Unlike the Savannah Hypothesis, one criterion for this may be that the last common ancestor (LCA) between chimpanzees and us used an arboreal form of bipedal locomotion (i.e., walking along branches using the arms for stability), similar to orangutans, and was not a knuckle-walker like contemporary chimpanzees.

The Evidence

Pollen evidence from the site of Makapansgat in South Africa indicated that around the time early hominins occupied the area, it was a closed, wooded environment. Similarly, the earliest evidence for bipedalism occurs in specimens (associated with taxa such as Orrorin and Ardipithecus spp. as well as Australopithecus anamensis) found in sites with evidence of closed habitats (Suwa et al. 2009). Furthermore, evidence of knuckle-walking in older hominin species is sorely lacking or highly contested.

Definition: closed habitat

A phrase typically referring to a woodland, or tree-filled, environment.

Interpretations

This hypothesis can be considered in contrast to the Savannah Hypothesis, and it does appear to be evidence based. However, it is worth noting that preservation and resulting fossilization might be better in these kinds of environments, biasing this interpretation of the fossil record. Evidence for knuckle-walking in our more distant ancestors is also highly contested.

Variability Selection Hypothesis

The Hypothesis

This hypothesis was first articulated by paleoanthropologist Richard Potts (1998). It links the high amount of climatic variability over the last 7 million years to both behavioral and morphological changes. Unlike previous notions, this hypothesis states that hominin evolution does not respond to habitat-specific changes or to specific aridity or moisture trends. Instead, long-term environmental unpredictability over time and space influenced morphological and behavioral adaptations that would help hominins survive, regardless of environmental context (Potts 1998; Potts 2013). The Variability Selection Hypothesis states that hominin groups would experience varying degrees of natural selection due to continually changing environments and potential group isolation. This would allow certain groups to develop genetic combinations that would increase their ability to survive in shifting environments. These populations would then have a genetic advantage over others that were forced into habitat-specific adaptations (Potts 2013).

The Evidence

The evidence for this theory is similar to that for the Turnover Pulse Hypothesis: large climatic variability and higher survivability of generalists versus specialists. However, this hypothesis accommodates for larger time-scales of extinction and survival events.

Interpretations

In this way, the Variability Selection Hypothesis allows for a more flexible interpretation of the evolution of bipedalism in hominins, accommodating the discrepancies in evidence between the conflicting Savannah and Forest Hypotheses. This also allows for a more fluid interpretation of the Turnover Pulse Hypothesis, where species turnover is meant to be more rapid. In some ways, this hypothesis accommodates both environmental data and our interpretations of an evolution toward greater variability among species and the survivability of generalists.

Pulsed Variability Selection Hypothesis

The Hypothesis

This hypothesis proposes that the East African Rift System (EARS) and changes in deep lakes are key drivers of diversification during early human evolution. EARS first developed about 10 mya and is responsible for the creation of large super lakes (e.g., Lake Baringo and Lake Turkana) within East Africa. The water contents of these lakes were primarily affected by both monsoonal rains and solar precessional cycles (cyclical changes in earth’s axis rotation- or wobble- that have global climatic effects). According to the Pulsed Variability Selection Hypothesis, human evolution was greatly affected by 200,000-year cyclical changes in aridity and humidity in this region, caused by those global cyclical changes.

Definition: solar precessional cycles

Cyclical changes in earth’s axis rotation that have global climatic effects.

The Evidence

 Proponents of this hypothesis name three extreme humid phases in East Africa at 2.7 mya to 2.5 mya, 1.8 mya to 1.6 mya, and 1 mya to 0.7 mya. During these periods, changes in solar precessional cycles increased the monsoonal system, causing more rain in East Africa, thereby increasing lake sizes. This is documented by the increase of diatomaceous lake sediments during these times. These expanded lakes would act as geographic barriers to hominin populations, allowing for changes and diversification in diet and adaptive behavior to the variable regions, even resulting in (allopatric) speciation (Maslin et al. 2014).

The Interpretation

High levels of species diversity during these time periods as well as environmental indications of these barriers may allow for an interpretation of allopatric-speciation (i.e., speciation due to geographic barriers) events. However, the degree of interspecific variability and the extent to which these barriers acted as drivers of speciation are still debated.

Paleoenvironment Consolidated Summary

Some of the hypotheses presented in this section pay specific attention to habitat (Savannah and Forest Hypotheses) while others point to large-scale climatic forces (Pulsed Variability and Variability Selection Hypotheses). Some are complementary (Pulsed Variability and Turnover Pulse Hypotheses), whereas others are directly opposed to one another (Savannah and Forest Hypotheses). Some may be interpreted to describe the evolution of traits such as bipedalism (Savannah and Forest Hypotheses), and others more generally explain the diversification of early hominins (Turnover Pulse and Variability Selection Hypotheses). While there is no consensus as to how the environment drove our evolution, it is clear that the environment shaped both habitat and resource availability in ways that would have influenced our early ancestors physically and behaviorally.

REFERENCES

Behrensmeyer, A. K., N. E. Todd, R. Potts, and G. E. McBrinn. 1997. “Late Pliocene Faunal Turnover in the Turkana Basin, Kenya, and Ethiopia.” Science 278 (5343): 637–640.

Bobe, R. and A. K. Behrensmeyer. 2004. “The Expansion of Grassland Ecosystems in Africa in Relation to Mammalian Evolution and the Origin of the Genus Homo.” Palaeogeography, Palaeoclimatology, Palaeoecology 207 (3–4): 399–420.

Cerling, T. E., J. G. Wynn, S. A. Andanje, M. I. Bird, D. K. Korir, N. E. Levin, W. Mace, A. N. Macharia, J. Quade, and C. H. Remien. 2011. “Woody Cover and Hominin Environments in the Past 6 Million Years.” Nature 476: 51e56.

Dart, R. A. 1925. “Australopithecus africanus, the Man-Ape of South Africa.” Nature 115: 195–199.

Darwin, Charles. 1871. The Descent of Man: And Selection in Relation to Sex. London: J. Murray.

DeHeinzelin, J., J. D. Clark, T. White, W. Hart, P. Renne, G. WoldeGabriel, Y. Beyene, and E. Vrba. 1999. “Environment and Behavior of 2.5-million-year-old Bouri Hominids.” Science 284 (5414): 625–629.

DeMenocal, P. B. D. 2004. “African Climate Change and Faunal Evolution during the Pliocene–Pleistocene.” Earth and Planetary Science Letters 220 (1–2): 3–24.

DeMenocal, P. B. D. and J. Bloemendal, J. 1995. “Plio-Pleistocene Climatic Variability in Subtropical Africa and the Paleoenvironment of Hominid Evolution: A Combined Data-Model Approach.” In Paleoclimate and Evolution, with Emphasis on Human Origins, edited by E.S. Vrba, G.H. Denton, T.C. Partridge, and L.H. Burckle, 262–288. New Haven: Yale University Press.

Faith, J. T. and A. K. Behrensmeyer. 2013. “Climate Change and Faunal Turnover: Testing the Mechanics of the Turnover-Pulse Hypothesis with South African Fossil Data.” Paleobiology 39 (4): 609–627.

Kingston, J. D. 2007. “Shifting Adaptive Landscapes: Progress and Challenges in Reconstructing Early Hominid Environments.” American Journal of Physical Anthropology 134 (S45): 20–58.

Kingston, J. D. and T. Harrison. 2007. “Isotopic Dietary Reconstructions of Pliocene Herbivores at Laetoli: Implications for Early Hominin Paleoecology.” Palaeogeography, Palaeoclimatology, Palaeoecology 243 (3–4): 272–306.

Maslin, M. A., C. M. Brierley, A. M. Milner, S. Shultz, M. H. Trauth, and K. E. Wilson. 2014. “East African Climate Pulses and Early Human Evolution.” Quaternary Science Reviews 101: 1–17.

Potts, R. 1998. “Environmental Hypotheses of Hominin Evolution.” American Journal of Physical Anthropology 107 (S27): 93–136.

Potts, R. 2013. “Hominin Evolution in Settings of Strong Environmental Variability.” Quaternary Science Reviews 73: 1–13.

Rayner, R. J., B. P. Moon, and J. C. Masters. 1993. “The Makapansgat Australopithecine Environments.” Journal of Human Evolution 24(3): 219–231.

Shipman, Pat. 2002. The Man Who Found the Missing Link: Eugene Dubois and his Lifelong Quest to Prove Darwin Right. New York: Simon & Schuster.

Suwa, G., B. Asfaw, R. T. Kono, D. Kubo, C. O. Lovejoy, and T. D. White. 2009. “The Ardipithecus ramidus Skull and Its Implications for Hominid Origins.” Science 326 (5949): 68–68e7.

Vrba, E. S. 1988. “Late Pliocene Climatic Events and Hominid Evolution.” In The Evolutionary History of the Robust Australopithecines, edited by F. E. Grine, 405–426. New York: Aldine.

Vrba, E. S. 1998. “Multiphasic Growth Models and the Evolution of Prolonged Growth Exemplified by Human Brain Evolution.” Journal of Theoretical Biology 190(3): 227–239.

Vrba, E. S. 2000. “Major Features of Neogene Mammalian Evolution in Africa.” In Cenozoic Geology of Southern Africa, edited by T. C. Partridge and R. Maud, 277–304. Oxford: Oxford University Press.

FIGURE ATTRIBUTIONS

Figure 9.1.1 IMG_1696 Great Rift Valley by Ninara is used under a CC BY 2.0 License.

Figure 9.1.2 Clades original to Explorations: An Open Invitation to Biological Anthropology by Katie Nelson is under a CC BY-NC 4.0 License.

Figure 9.1.3 CO₂ increase since the Industrial Revolution by NASA, original from Luthi, D., et al.. 2008; Etheridge, D.M., et al. 2010; Vostok ice core data/J.R. Petit et al.; NOAA Mauna Loa CO₂ record is in the public domain and used within NASA guidelines on re-use.

Figure 9.1.4 African savannah @ Masai Mara (21308330314) by Leo Li from Hong Kong is used under a CC BY 2.0 License.