Homo erectus is arguably the earliest species in the human lineage to have so many human-like qualities. Earlier hominins had important similarities with living humans, like bipedality, and H. erectus still had a long evolutionary path to become like you and me, but the fossils assigned to H. erectus display a number of new and distinctly modern human traits.

Homo erectus is often referred to as the first cosmopolitan hominin lineage, meaning the first hominin species whose geographic range had expanded beyond a single continental region. While fossil remains from H. erectus are found in Africa, like those of earlier hominins, they have also been identified at fossil sites widely dispersed across Eurasia (Figure 1, Table 1).

Date (mya)	Locality	Key Fossils
1.9 – 1.2	Koobi Fora, Kenya	WT 15000 (Nariokotome), ER-3733, ER-3883
1.9 – 0.7	Olduvia Gorge, Tanzania	OH 9, OH 12
1.8 – 1.7	Dmanisi, Georgia	D3444, D2700, D2280, D2282
1.8 – 1.6	Swartkrans, South Africa	SK 847
1.8 – 0.9	Sangiran/Trinil, Indonesia	Trinil 2, Mojokerto, Sangiran 17, Sangiran 2
1.0 – 0.8	Ceprano, Italy	Ceprano 1
0.8 – 0.4	Zhoukoudian, China	ZKD E1, D1, L1, L2, H3
0.8 – 0.6	Bodo, Ethiopia	Bodo
0.6 – 0.3	Atapuerca, Spain	Sima de los huesos (numerous)
0.3 – 0.1	Jinniushan, China	Jinniushan
0.2 – 0.05	Ngandong, Indonesia	Ngandong 1, 9, 10, 11
Table 1: Key Homo erectus fossil sites. A partial list of key Homo erectus fossil localities, and some of the key specimens preserved at each. Exact dates are difficult to obtain for many of these localities, so the above dates represent best approximate ranges. In some cases, such as Olduvai Gorge and Koobi Fora, fossils have been recovered from many individual localities within the area, spanning a large range of dates.

There are a number of fascinating evolutionary questions that can be asked of H. erectus. The species was not only geographically widespread, it also had a long temporal span in the hominin fossil record (Antón 2003). With its earliest appearance in the fossil record from localities in the Lake Turkana Basin, Kenya, sometime around two million years ago, H. erectus populations persisted until near the end of the Pleistocene, as evidenced by fossils from Southeast Asia. Homo erectus thus presents paleoanthropologists with the challenge of trying to interpret fossil variation in the context of both widespread geographic and temporal distribution.

Furthermore, the expansion of H. erectus across a large range of environments suggests a change in the ecology of this lineage relative to early hominins, a change that certainly has significance for how evolutionary forces acted to shape the pattern of variation we observe in the fossil lineage.

These are some of the questions that researchers ask of H. erectus fossils: How did the ecology of Homo erectus differ from that of preceding hominins? What are the characteristics of H. erectus that allowed it to expand across different habitats throughout portions of Eurasia and Africa? What limitations constrained the expansion and evolution of H. erectus in the Pleistocene? What role did behavioral and technological innovation play in establishing the complex and geographically widespread evolutionary pattern of H. erectus? How might we describe and explain the evolutionary pattern of H. erectus?

History and Geography

Eugene Dubois first identified and described a new human-like set of Indonesian fossils at the end of the 19^th century, naming the specimens Pithecanthropus erectus (upright, ape-man) because of their combination of bipedality and a brain size much smaller than living humans. Dubois had specifically been looking for the missing link between apes and humans, and for him the combination of a human-like body and ape-like brain represented just that (Shipman 2002). Subsequent discoveries in the 1920s and 1930s from the site of Zhoukoudian, China, of fossils with similar characteristics-originally designated Sinanthropus pekinensis-raised the question of a possible evolutionary relationship between these regional samples. Today, these two samples, along with a much larger collection of fossils from Asia, Africa, and Europe, are most commonly referred to simply as Homo erectus.

What is the evolutionary relationship among fossils that share many similarities, but also subtle differences, separated across time and space? This question, prompted by the early Chinese and Javan fossils, remains an active research question today for the much larger sample of fossils assigned to H. erectus. Whether or not a sample from one region, for example Africa, part of a polytypic, geographically widespread lineage (Homo erectus), or whether it is part of a related, but different species, is a debated topic and reveals much about the evolutionary pattern of the species (Rightmire 1998). For example, some researchers argue that H. erectus is restricted largely to Eastern and Southeast Asia, consistent with the original fossils attributed to the taxon. In that case, the bulk of its representatives lived from the end of the Lower Pleistocene through the Middle Pleistocene (~1.4-0.2 mya). From this perspective, earlier fossils from Western Asia (e.g., Dmanisi, Georgia; Figure 2) and Africa (e.g., Koobi Fora, Kenya) that are similar to the classic Asian H. erectus, but also have more ancestral traits, might be considered a separate lineage (often called Homo ergaster). Middle Pleistocene remains from Europe might be a second or third separate lineage (Homo heidelbergensis). In this view, the ecological niche occupied by these species is more limited, leading to the isolation, and ultimately speciation, among different regional populations.

Humans are widespread and variable today, but much of the variation observed across contemporary populations is the result of relatively recent events in the past 100,000 years of our evolutionary history. Patterns of variation in H. erectus occurred on a time scale as long as a million years, and may have been different from those we observe today. This presents a challenge for researchers in terms of how we explain the pattern of variation seen in H. erectus, but also presents an opportunity to study how evolutionary forces operate across such scales.

Who was Homo erectus?

As with any fossil lineage, identifying the earliest appearance of the species is difficult. Nevertheless, a set of shared, derived features can be assigned to all of the fossils assigned to H. erectus, regardless of where they fall amid the geographic and temporal range of the lineage. The following sections summarize some of these characteristics.

One of the traits most commonly associated with Homo erectus is an increase in body size. The Nariokotome specimen, an adolescent male individual, was over five feet tall at the time of his death (Walker & Leakey 1993; Figures 3 & 4). Estimates of his adult stature, had he continued to live to adulthood, differ depending on how researchers estimate his age at death based on his teeth and bones (for more see Smith & Alemseged NKP article LINK HERE) and the amount of growth he had left. Living humans generally experience a marked increase in growth during early adolescence (i.e., a ‘growth spurt'), a growth pattern that some researchers say distinguishes us from other apes. If early H. erectus had a human-like growth spurt, Nariokotome likely had a lot of growing left to do. Even if H. erectus did not have such a modern human-like pattern of growth, the specimen was clearly a tall individual relative to earlier hominins. Not all H. erectus were tall, however, as earlier fossils remains from sites like Olduvai Gorge in Tanzania and Dmanisi, Georgia, attest. It is important to note that variations in size, not just an increase in size over that of earlier hominins, is characteristic of H. erectus, much like living humans.

As the body size of hominins increases, the brain size increases as well (Ruff et al. 1997). While the smallest-bodied early H. erectus fossils have brain sizes only slightly larger than earlier hominins (australopiths), early large-bodied specimens, such as the Nariokotome individual, have a brain volume greater than 800 cm³, more than 50% larger than earlier australopiths (and about 60% of the typical brain size of someone living today). However, in addition to the absolute increase in brain volume that accompanies an increase in body size, there is also a proportional increase. This is referred to as encephalization, and is an important characteristic of H. erectus. Throughout the evolutionary history of H. erectus there is substantial evidence for selection leading towards increased encephalization, so that while early members of the lineage have a cranial capacity of 600-800 cm³, the cranial capacities of most later specimens are well in excess of 1000 cm³, which is within the lower range of contemporary humans, without appearing considerably larger in body size than early H. erectus.

The large body and large brain of H. erectus needed more energy, and thus food, than previous hominins. Larger biological structures, particularly energy-intensive ones like muscles and brains, require greater energy inputs to maintain. Thus, H. erectus is often reconstructed as occupying an intensified ecological niche (Leonard & Robertson 1997).

The intensified niche goes hand in hand with the expansion in brain and body size. Larger bodies, and longer limbs in particular, increase locomotor efficiency (Pontzer et al. 2010). Homo erectus could cover more ground on a day-to-day basis, through walking or running, than smaller hominins and with lower energy cost. In addition, the larger brain gave these hominins better capabilities for processing complex ecological information across the more expansive terrain containing higher quality food items. For example, there is clear evidence of H. erectus accessing medium- and large-sized animal carcasses for meat, through hunting and/or scavenging, in the form of fossil remains of animals with cut marks left by butchery. This behavior, regularly accessing animal carcasses, is an ecological change from earlier hominins (Link to Pobiner's NKP article). While the earliest H. erectus specimens are found in association with very basic stone tools, typically referred to as the Oldowan stone tool industry, by 1.5 million years ago populations of H. erectus were creating a more complex and typologically codified set of tools that we refer to as the Acheulean industry (Bar-Yosef & Belfer-Cohen 2001).

The change in ecology associated with H. erectus coincides with a corresponding change in the dentition and jaws of this species. Relative to earlier australopiths and contemporary robust australopiths (Paranthropus), the size of the post-canine dentition (premolars and molars) and the molarization of the premolars are dramatically reduced in H. erectus. The corpus of the mandible (i.e., the toothless part that connects to the cranium) also displays increased gracility (i.e., slenderness), with a characteristic reduction in the relative breadth of the structure and supportive masticatory structures. Toothwear analyses suggest that across their range, H. erectus engaged in a diverse and broad diet (Ungar et al. 2006). The food an organism ingests can also produce subtle changes in the chemistry of body tissues (you actually are what you eat), including the dentin and enamel that make up the crown of a tooth. Using this information, investigations of the stable isotope chemistry of H. erectus teeth also support the idea of a flexible and diverse diet (Ungar & Sponheimer 2011). Whatever the flora and fauna H. erectus ate at the varied geographic localities where H. erectus fossils are found, their tooth and jaw anatomy reveal that their diet did not require the same robust masticatory adaptations seen in earlier hominins.

Unanswered Questions About Homo erectus

Two of the more significant, yet elusive, questions about H. erectus concern the levels of sexual dimorphism within the lineage and the capacity for language. Sexual dimorphism, the physical differences between males and females, is an important source of variation within species, and in primates can be an indicator of reproductive strategy and group dynamics. Sexual dimorphism, given its role in intraspecific variation, can also be a confounding factor in proper taxonomic identification. The large amount of size variation observed within H. erectus, taken primarily from fragmentary fossil remains, makes it difficult to estimate average levels of dimorphism. The H. erectus fossil record provides clear evidence of a large range of skeletal size variation, at least equivalent to that observed in living human populations, but it does not provide conclusive evidence that males were systematically larger than females to a greater extent than they are today. If H. erectus did have more sexual dimorphism than H. sapiens, we would infer that male competition for mates was more dependent on body size than it is today.

Language is perhaps the hallmark human trait, but can be difficult to assess directly from the fossil record. Attempts to identify language ability in the fossilized skeletal remains of H. erectus have focused on aspects of the nervous system, including the size of the vertebral canal (a proxy for the size of the spinal cord), and external features of endocasts (natural fossils of endocranial space and a proxy for brain size and shape). Thus far, there have been no definitive anatomical findings to cause researchers to reject the idea that H. erectus was capable of some kind of human-like proto-language.

More recently, arguments about the origins of language have focused on the reconstructed histories of genes associated with language production in humans. The recovery of ancient genetic sequences from Neandertals and other archaic human specimens (e.g., a specimen from Denisova Cave in Siberia, Russia) have provided new insight into the genetic history of language production. The human FOXP2 gene exists in a derived form in humans today and appears to play a critical role in language development. The identification of the human form of FOXP2 in both Neandertal and Denisovan genomes suggests this gene likely goes back at least to the Middle Pleistocene, with H. erectus a possible source lineage (although there is no H. erectus ancient DNA to test this hypothesis). This does not suggest H. erectus had well-developed language capabilities but, like the anatomical evidence, does not provide any evidence to reject the idea.

Summary

Homo erectus represents a significant transformation from previous hominins, like the australopiths, to a species much more similar to modern humans. Relative to their australopith forebears, Homo erectus was bigger, smarter, and more able to occupy and survive in differing landscapes in a changing world. The movement towards a more ecologically intense, cognitively reliant, and behaviorally malleable adaptive pattern set the stage for the evolutionary change that followed in the Pleistocene, up to and including the present. In many ways, modern humans are just an updated version of our H. erectus ancestors.

Acheulean - Lower and Middle Pleistocene hominin stone tool industry. The Acheulean tool complex is often characterized by a high percentage of bifacially flaked stone cores and the presence of tear-drop shaped tools referred to as ‘hand axes'.

ancestral - A trait that is present in the common ancestor of a species. The large body size of contemporary humans is ancestral, as evidenced by the presence of this feature in Homo erectus.

derived - A trait that is not present in the common ancestor of a species, but is newly arisen. The marked encephalization of Homo erectus is a derived characteristic relative to earlier, small-brained hominins.

ecological niche - The overall set of relations that defines the place of a species within its environment. This includes the other organisms a species interacts with, such as prey or predators, as well as the physical habitats a species utilizes in its existence.

endocasts - A natural fossil cast formed within the endocranial space of a skull. When present, endocasts provide some resolution on the size, shape, and surface structures of the brain in fossil taxa.

encephalization - Expansion of the brain relative to body size. Encephalization represents an increase in proportional resources dedicated to the growth, development, and maintenance of brain activities.

masticatory - Of, or relating to, the chewing structures of an organism.

Oldowan - The earliest well-characterized hominin stone tool industry, present in the terminal Pliocene and Lower Pleistocene. This tool complex is characterized by a simple core-flake set of tools.

polytypic - The presence of multiple forms of a lineage across a species' range.

sexual dimorphism - The characteristic differences between males and females within a species. Often this refers specifically to size sexual dimorphism, the average difference in body mass or skeletal size between males and females of a species.