Beginning of summer, now is as good a time as any to clear some space on my desktop and finally get rid of all the Firefox tabs I keep open, firmly believing I will have time to read them, soon. I’ve decided to try and go through the scientific articles that I have accumulated in the past months, and I will probably write about a few of them along the way.
Among the first ones on the top of the pile is a review published in Science on July 4th (not that long ago; I will slowly walk my way back in time). The paper, by Antón and colleagues, is a scientific review about the evolution of early Homo. As any review, it goes over the scientific literature available on the topic, taking into account existing data, previous hypotheses and interpretations, and recent advances, and weaves it into an account of what is currently known and unknown.
Paleoanthropology is certainly not my area of expertise, but once in a while I like to read a paper on the subject and learn a little something new. So, what have I (re)learned here?
1. It’s not the straight line that one imagines as a kid.
Well, I knew that already. However it does not hurt to become reacquainted from time to time with how complicated the real picture is, and how difficult it is to resolve it.
Thinking about the evolution of human beings, most people have one representation in mind: this one picture one sees everywhere, of an ape progressively straightening up to become a human being, as if human evolution was a succession of neatly separated species existing one after the other, with traits steadily evolving in one direction towards those of modern human beings. Of course, that is not correct. A better picture would be of multiple species/genera, some co-existing over (relatively) long periods of time, with overlapping characteristics as well as variations between and within species, and with interrogations remaining as to transitions between/within lineages.
2. Three types of hominins co-existing in Africa around 2.5-2.0 million years ago.
The earliest specimen attributed to the genus Australopithecus dates from about 4 million years ago (mya). Lucy, perhaps the most famous fossil hominin, belongs to the species Australopithecus afarensis and is dated at about 3.2 mya. Lucy was found in Northeastern Ethiopia, at a site called Hadar. A specimen of Australopithecus sediba, recently discovered in South Africa and dated at about 1.98 mya, is now the focus of much interest as a potential link to the beginnings of the genus Homo, because of some its Homo-like anatomical features.
Specimens of the genus Paranthropus are found in the fossil record between about 2.5 and 1.4 mya. This genus seems to have co-existed with the early members of the genus Homo for about a million years. (Some researchers place the Paranthropus fossil specimens in the genus Australopithecus, referring to them as the robust Australopithecines.)
One of the earliest specimens of the genus Homo to be found in the fossil record is dated at about 2.33 mya. It was found at the Hadar site in Ethiopia.
3. How many early Homo species?
Paleoanthropologists have traditionally divided specimens of early Homo in up to four species: Homo habilis, Homo rudolfensis, Homo ergaster, and Homo erectus. This classification is based on anatomical features that were deemed to be too different to represent members of a single species. (In addition, fossils are scattered across time and space, which makes it even more complicated to distinguish variation between species from variation between populations of the same species that may have existed across different time periods and/or geographical areas.)
Classification and name-giving have of course never been a straightforward matter, and paleoanthropologists sometimes diverge in their opinions of what species fossil specimens should be attributed to. Some have even wondered if early Homo specimens might not all be collapsed into one single evolving lineage of Homo erectus, mostly based on an analysis of the Dmanisi fossils.
Given the evidence currently available, the authors of the Science review favor the hypothesis that there were three distinct lineages of early Homo existing in Africa about 2 mya. However, they do not fully adhere to the classical categorization of specimens into H. habilis and H. rudolfensis (besides H. erectus), and they instead propose to use two slightly different groups called 1470 and 1813 after their most well-known characteristic specimens.
Regardless of what they are called, these three species of early Homo partly overlap in the fossil record (from about 2 to 1.5 mya), and they all display, on average, larger brains and bodies than specimens of the genus Australopithecus.
3. Variable climate, savanna grasslands and woodlands
What I remember from my school years is that the genus Homo appeared within a context of changing environment in Africa, from woodlands to open savanna grasslands, as the climate moved towards greater aridity. However, as all kinds of data have accumulated (analyses of eolian dust, volcanic and tectonic activity, lake areas, fauna, flora, etc.), scientists have realized that the African climate and landscape around the time early Homo evolved was far more fluctuating than previously thought. There was indeed a global trend of cooling and developing aridity between about 3 and 1.5 mya in East Africa, but there were also periods of high environmental instability, with alternating periods of aridity and intense moisture. The landscape was probably made of savanna habitats with a variable degree of wood cover (from 5 to 80%).
The authors of the Science review argue that this highly fluctuating environment played an important role in the evolution of the genus Homo: it defined an adaptive landscape in which traits and behaviors allowing for successful accommodation of novel and changing environments (rather than specializing in living in one kind of environment) were favored.
4. Not a single package
The anatomical and behavioral traits traditionally used to characterize the origin of the genus Homo were once thought to have appeared as a “single package”, but new fossil and archeological data, as well as models based on comparative biology and behavioral ecology, indicate that these traits evolved over a prolonged period of time, encompassing species of Australopithecus, early Homo, Homo erectus, and later Homo. To name a few:
– bipedality: bipedality evolved early in the hominin lineage, well before the origin of the genus Homo. Anatomical features of early hominins such as Sahelanthropus tchadensis (6-7 mya), Orrorin tugenensis (around 6 mya), and Ardipithecus ramidus (4.4 mya) show evidence of some bipedality, although they also retain characteristics allowing for efficient locomotion in trees. By contrast, the Australopithecines (appearing around 4 mya in the fossil record) were obligate bipeds, meaning that they had anatomical features that were specialized for efficient bipedal walking, to the detriment of other kinds of locomotion (for example, the first toe is in line with the other toes in Australopithecus, whereas Ardipithecus still had a grasping first toe).
– stone tools: evidence of tool use in the fossil record appears around 2.6 mya, possibly predating the genus Homo; the Oldowan stone tool technology (cores and flakes) may have been used by late Australopithecines and early Homo and appears more persistently in the fossil record from about 2 mya. Acheulean stone tools (large cutting tools, hand axes) and controlled use of fire followed, at around 1.7 mya.
– dietary expansion: there is evidence of an initial dietary expansion in both the Australopithecus and Homo lineages between 4 and 3 mya (diversification in the kind of plant products they relied on), with further expansion of the diet and inclusion of large animals and underground plants in early Homo around 2 mya.
– increase in body and brain size: both body and brain size increased on average from Australopithecus to early Homo and Homo erectus, though estimates of the degree of brain enlargement correlated with body size in early Homo erectus (1.9-1.5 mya) overlap with those in Australopithecus. A highly encephalized brain (brain expansion independently of body size) developed mostly later, between 800,000 to 200,000 years ago.
5. Homo erectus went far away from Africa, a long time ago.
Current fossil evidence suggests that Homo erectus was the first hominin to leave Africa, spreading across Eurasia around 1.9-1.7 mya.
The field of paleoanthropology is an active and diverse one, where there is much to be explored and refined with not only each new fossil find, but also with environmental and ecological studies, as well as comparative biological studies. The Science review touched upon other aspects of the evolution of early Homo that I have not relayed here, such as life history, development, cognitive evolution, and energetics, but for whoever is interested in knowing more about the history of the hominins, there are many great resources out there.
To cite only two:
Antón SC, Potts R, & Aiello LC (2014). Human evolution. Evolution of early Homo: an integrated biological perspective. Science (New York, N.Y.), 345 (6192) PMID: 24994657