We are discovering that the evolution of the genus Homo is more complex than what was previously thought. The earlier view of a simple progression from Australopithecus to Homo habilis to Homo erectus as clearly delineated stages in human evolution just doesn’t hold up anymore.
As is apparent from the information presented here, there is tremendous variability during this time. While fossils classified as Homo habilis show many of the characteristics of the genus Homo, such as brain expansion and smaller tooth size, the small body size and long arms are more akin to australopithecines. There is also tremendous variability within the fossils assigned to Homo habilis, so there is little consensus on whether it is one or multiple species of Homo, a member of the genus Australopithecus, or even a yet-to-be-defined new genus. Similarly, there are considerable differences in skull morphology and body size and form of Homo erectus, of which some specimens show more similarity to Homo habilis than previously thought.
What does this diversity mean for how we should view early Homo? First, there isn’t an abrupt break between Australopithecus and Homo habilis or even between Homo habilis and Homo erectus. Characteristics we define as Homo don’t appear as a unified package; they appear in the fossil record at different times. This is known as mosaic evolution. Indeed, fossil species such as Australopithecus sediba, as well as Homo naledi and Homo floresiensis (who will be introduced in Chapter 11), have displayed unexpected combinations of primitive and derived traits.
We can consider several explanations for the diversity we see within early Homo from about 2.5 million to 1.5 million years ago. One possibility is the existence of multiple contemporaneous species of early Homo during this period. In light of the pattern of environmental instability discussed earlier, it shouldn’t be surprising to see fossils from different parts of Africa and Eurasia display tremendous variability. Multiple hominin forms could also evolve in the same region, as they diversified in order to occupy different ecological niches. However, even the presence of multiple species of hominin does not preclude their interacting and interbreeding with one another. As you’ll see in Appendix D, sequencing of ancient hominin genomes has led to deeper understanding of genetic relationships between extinct species such as the Neanderthals and Denisovans.
Diversity of brain and body sizes could also reflect developmental plasticity—short-term adaptations within a lifetime (Anton et al. 2014). These have the advantage of being more flexible than genetic natural selection, which could only occur over many generations. For example, among human populations today, different body sizes are thought to be adaptations to different climate or nutritional environments. Under Pleistocene conditions of intense variability, a more flexible strategy of adaptation would be valuable.
New discoveries are also questioning old assumptions about the behavior of Homo habilis and Homo erectus. Just as the fossil evidence doesn’t neatly separate Australopithecus and Homo, evidence of the lifeways of early Homo show similar diversity. For example, one of the traditional dividing lines between Homo and Australopithecus was thought to be stone tools: Homo made them; Australopithecus didn’t. However, the recent discovery of stone tools from Kenya dating to 3.3 million years ago challenges this point of view. Similarly, the belief that Homo erectus was the first species to settle outside Africa may now come into question with the report of 2.1-million-year-old stone tools from China. If this find is supported by additional evidence, it may cause a reevaluation of Homo erectus being the first to leave Africa. Instead, there could have been multiple earlier migrations of hominins such as Homo habilis or even Australopithecus species.
These various lines of evidence about the genus Homo point out the need for a more nuanced view of this period of human evolution. Rather than obvious demarcations between species and their corresponding behavioral advancements, it now looks like many behaviors were shared among species. Earlier hominins that we previously didn’t think had the capability could have been doing things like expanding out of Africa or using stone tools. Meanwhile, some other hominins that we had considered more advanced didn’t actually have the full suite of “human” characteristics previously expected.
From a student’s perspective, all this complexity probably seems frustrating. It would be ideal if the human story were a straightforward, sequential narrative. Unfortunately, it seems that human evolution was not a nice, neat trajectory of increasingly humanlike traits and behaviors; rather, it is emblematic of the untidy but exciting nature of the study of human evolution.
Despite some haziness dominating the early Homo narrative, we can identify some overall trends for the million-year period associated with early Homo. These trends include brain expansion, a reduction in facial prognathism, smaller jaw and tooth size, larger body size, and evidence of full terrestrial bipedalism. These traits are associated with a key behavioral shift that emphasizes culture as a flexible strategy to adapt to unpredictable environmental circumstances. Included in this repertoire are the creation and use of stone tools to process meat obtained by scavenging and later hunting, a utilization of fire and cooking, and the roots of the human life history pattern of prolonged childhood, cooperation in child raising, and the practice of skilled foraging techniques. In fact, it’s apparent that the cultural innovations are driving the biological changes, and vice versa, fueling a feedback loop that continues during the later stages of human evolution.
Hominin Species Summaries
| Hominin |
Homo habilis |
| Dates |
2.5 million years ago to 1.7 million years ago |
| Region(s) |
East and South Africa |
| Famous discoveries |
Olduvai Gorge, Tanzania; Koobi Fora, Kenya; Sterkfontein, South Africa |
| Brain size |
650 cc average (range from 510 cc to 775 cc) |
| Dentition |
Smaller teeth with thinner enamel compared to Australopithecus; parabolic dental arcade shape |
| Cranial features |
Rounder cranium and less facial prognathism than Australopithecus |
| Postcranial features |
Small stature; similar body plan to Australopithecus |
| Culture |
Oldowan tools |
| Other |
N/A |
| |
|
| Hominin |
Homo erectus |
| Dates |
1.8 million years ago to about 110,000 years ago |
| Region(s) |
East and South Africa; West Eurasia; China and Southeast Asia |
| Famous discoveries |
Lake Turkana, Olorgesailie, Kenya; Java, Indonesia; Zhoukoudian, China; Dmanisi, Republic of Georgia |
| Brain size |
Average 900 cc; range between 650 cc and 1,100 cc |
| Dentition |
Smaller teeth than Homo habilis |
| Cranial features |
Long, low skull with robust features including thick cranial vault bones and large brow ridge, sagittal keel, and occipital torus |
| Postcranial features |
Larger body size compared to Homo habilis; body proportions (longer legs and shorter arms) similar to Homo sapiens |
| Culture |
Acheulean tools (in Africa); evidence of increased hunting and meat-eating; use of fire; migration out of Africa |
| Other |
N/A |
