The Evolution of Cranial Capacity in Humans and Stem-Humans

(Hey, I'm actually writing on the blog's title subject today!)

Here's a chart I've been working on for a while:

This shows all known human and stem-human individuals, plotted according to stratigraphy and cranial capacity (endocranial volume). The fossil individuals with known cranial capacity are highlighted as white circles; other fossil individuals' probable capacity is inferred from these. The "chimpanzee range" shows the span between a normal female bonobo chimpanzee (Pan paniscus) and a normal male common chimpanzee (Pan troglodytes) (Begun & Kordos 2004); the full range for chimpanzees (Pan) is slightly larger (but not much). The "human range" shows where about 90% of living humans fall (Burenhult 1993). UPDATE: My mistake, it's the range of ~90% of living humans combined with the range of Upper Pleistocene humans (which is actually higher, on average).

Some notes:

• The Upper Pleistocene (Tarantian) and the Holocene (Recent) are too small to label.
• In hominids (great apes and humans), variation in body size accounts for some variation in brain size, but not all of it.
• In all hominid species, females have a lower average cranial capacity than males.
• The area for "Homo habilis & rudolfensis" includes a very early specimen from Hadar, Ethiopia referred to Homo aff. habilis. Without that, the lowest bound for the area wouldn't be far past the lowest bound for Homo ergaster.
• A single specimen, Skull 5 from Dmanisi, Georgia, causes Homo ergaster to overlap with Australopithecus & Paranthropus. Some researchers place the Dmanisi specimens in their own species, Homo georgicus. Others use a very broad Homo erectus which encompasses everything I've placed in H. ergaster.
• The only two pre-Piacenzian skulls with known capacity are a female Ardipithecus ramidus ("Ardi") and a male(???) Sahelanthropus tchadensis ("Toumaï").
• Some cranial specimens do not have stratigraphic information. Most of these are modern humans (Homo sapiens sapiens) and have been placed alonside other modern humans. One is a Neandertal (Homo neanderthalensis) with primitive traits from Biache-Saint-Vaast, France, and another is a Homo erectus from Ngawi, Java, with similarities to other Homo erectus specimens from Java (Ngandong and Sambungmacan localities). These have provisionally been stratigraphically placed near specimens similar to them.
• Many Australopithecus & Paranthropus specimens are from caves, and therefore difficult to place stratigraphically. This is why there are long vertical "streaks" in that area.
• In general we see some some clear trends. Stem-humans began in the Messinian with brains slightly smaller than those of living chimpazees, and reached chimpanzee size by the middle of the Pliocene. Some Piacenzian (Upper Pliocene) specimens even exceeded the chimpanzee range. In the Lower Pleistocene there was a split: Australopithecus and Paranthropus persisted with brains little larger than those of chimpanzees, while brain size positively exploded in Homo. Early species (Homo habilis & rudolfensis) still overlapped with chimpanzees, but on average had much larger brains. The slightly later Homo ergaster was almost entirely outside the chimpanzee range, and just barely into the human range. Homo erectus & heidelbergensis continued the trend, with brains generally becoming larger over time until they nearly matched the human range. Neandertals (Homo neanderthalensis) even exceeded the human range (although mostly overlapped it).
• The one major anomaly: Floresian "hobbits" (Homo floresiensis), a tiny species known from the Upper Pleistocene of Flores Island, Indonesia. Only one skull is complete enough to measure the capacity (Liang Bua 1), and it is at the low end of the chimpanzee range (380cc). This species has been suggested as a small-brained offshoot of Homo erectus, but the only cladistic analysis that has included it (Argue & al. 2009) has it branching off the human lineage before Homo ergaster or H. erectus.

References

Argue, Morwood, Sutikna, Jatmiko & Wayhu Saptomo (2009). Homo floresiensis: a cladistic analysis. J. Hum. Evol. 57:623–639. doi:10.1016/j.jhevol.2009.05.002

Begun & Kordos (2004). Cranial evidence of the evolution of intelligence in fossil apes. Pages 260–179 in Russon & Begun (eds.) The Evolution of Thought: Evolutionary Origins of Great Ape Intelligence. Cambridge University Press.

Berger, de Ruiter, Churchill, Schmid, Carlson, Dirks & Kibii (2010). Australopithecus sediba: a new species of Homo-like australopith from South Africa. Science 328(5975):195–204. doi:10.1126/science.1184944 

Brown, Sutikna, Morwood, Soejono, Jatmiko, Wayhu Saptomo & Rokus Awe Due (2004). A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature 431:1055–1061. doi:10.1038/nature02999

Brunet, Guy & [36 others] (2002). A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145–151. doi:10.1038/nature00879

Burenhult G. (1993). The First Humans: Human Origins and History to 10,000 B.C. HarperCollins, New York.

Holloway, Broadfield & Yuan (2004). The Human Fossil Record: Brain Endocasts: The Paleoneurological Evidence. John Wiley & Sons, New York.

Lordkipanidze, Ponce de León, Margvelashvili, Rak, Rightmire, Vekua & Zollikofer (2013). A complete skull from Dmanisi, Georgia, and the evolutionary biology of early Homo. Science 342(6156):326–331. doi:10.1126/science.1238484