Although the body cylinder does not differ among Middle Pleistocene fossils from Africa and Europe, an important brain expansion does occur by at least 500 kya, leading to a clear increase in encephalization. The range of cranial capacities from the Sima de los Huesos varies from 1,100 to 1,390 cm3. Combining body size and shape and absolute and relative brain size, these Middle Pleistocene fossils represent a different morphotype from that of their Early Pleistocene ancestors as well as from that of Asian H. erectus. Those from Zhoukoudian in China have been dated to between 300 and 550 kya (Grün et al., 1997), but new dates using a different technique yielded much older results (~780 kya), at least for the lower levels (Shen et al., 2009). Thus, it is possible that by 500 kya, H. erectus survived only in Indonesia. The Neanderthals and modern humans show the highest encephalization because the increase in brain size is coupled with a decrease in body size, although by two different means. The Neanderthals underwent a shortening of the distal segments of the extremities, whereas H. sapiens shows a narrowing of the body cylinder.
In the middle of the Early Pleistocene at least two cranial and postcranial combined morphotypes coexisted, whereas during the late Middle and Late Pleistocene four morphotypes coexisted: that of Neanderthals, that of modern humans, the fossils from Ngandong (smaller brain size and assigned to a late population of H. erectus), and the australopithecine morphotype of H. floresiensis. If in the Late Pleistocene, when the fossil record is more complete, we find that four different human lineages coexisted, why not think that this has been the general trend?
It is important to point out here that, although this encephalization can be represented as a curve, it does not necessarily imply a steady, continuous rate of increase through time. In fact, body size, which is one of the variables involved in calculating the encephalization quotient, shows long periods of stability for each morphotype. Gould and Eldredge (1993) warn: “We have learned as a received truth of evolution, for example, that human brain size increased at an extraordinary (many say unprecedented) rate during later stages of our lineage. But this entrenched belief may be a chimera born of an error in averaging rates over both punctuations and subsequent periods of stasis.” Hominid taxonomy within the genus Homo could be refined further if body size and shape and brain size were considered along with craniodental features.
To approach a cranial analysis, I adopt a paleontological species definition based on an operational criterion: Two or more populations represent different paleontological species if the variation between them is clearly larger than the variation within each of them. Under these circumstances