A close analysis of 1,523 fossil elements dating back 430,000 years and belonging to a minimum of 19 individuals from a single population has revealed that the human body went through around four distinct stages during evolution. The fossils were discovered in Sima de los Huesos (SH) in Spain – a Middle Pleistocene site with the largest collection of postcranial skeletons ever found.
The study was published on August 31 in the journal Proceedings of the National Academy of Sciences (PNAS).
Juan Luis Arsuaga of the Centro Mixto Universidad Complutense de Madrid and colleagues examined the entire fossil collection using raw values for key skeletal parts as proxies to estimate stature, body breadth, and weight, and then compared the results to data obtained from other early human fossils.
From this, the research team was able to determine that the SH humans were relatively tall, wide and muscular – all characteristics they shared in common with the later Neanderthals, except that the latter had significantly larger brains. This finding suggests that the classic Neanderthal features evolved in a mosaic pattern, wherein some body parts precede others, as opposed to change happening all at once.
The study also provides evidence that before the advent of Homo sapiens, the human body remained relatively unchanged for around a million years.
“This is really interesting since it suggests that the evolutionary process in our genus is largely characterized by stasis (i.e. little to no evolutionary change) in body form for most of our evolutionary history,” said study co-author and Binghamton University anthropologist Rolf Quam.
After comparing the SH collection to the rest of the human fossil record, the researchers concluded that the human body went through four main evolutionary stages depending on the degree of arboreality (living in trees) and bipedalism (walking upright), with the SH population (together with earlier hominins and the later Neanderthals) representing the third stage, and modern humans (MH) – which are much narrower, less muscular and more encephalized – being the latest, fourth stage.
“In sum,” concluded the researchers, “SH offers the best proxy for the general postcranial size and shape of Homo for at least the past 1 million years until the appearance of MH. Despite large periods of morphological stasis in the general body plan, the anatomical details of the postcranial skeleton, as revealed in the SH sample, offer the best evidence for a pattern of mosaic evolution in the postcranium within the Neanderthal lineage.”