Evolution Update

Evolution Update

Insights into Human Anatomical Evolution from The "Pit of Bones"

Brandon Kieft September 1, 2015

An international research team recently published their analysis of fossils from the largest single collection of human remains in the world: a Spanish cave named Sima de los Huesos, or Pit of Bones.

In recent history, early human fossil, artifact, and DNA recovery have provided several clues about the early evolution of hominids. Notably, we have learned that the events and interactions leading up to the Homo sapien dominated planet of today were very complex.

Despite this wealth of knowledge, there are still many disputes and uncertainties concerning early Homo evolution. A recent study published in the Proceedings of the National Academy of Sciences by a large international collaboration of scientists hoped to quell some of this uncertainty by analyzing nearly 7,000 human fossils dating to ~430,000 years ago.

These fossils were excavated from a cave in the Atapuerca Mountains of Spain known as Sima de los Huesos, or Pit of Bones. Importantly, all the fossils used in the study were very likely to have originated at the site, making their comparison and description much more robust in contrast to existing comparative studies that often span large geographical distances.

The researchers used the postcranial bones (all skeletal bones except the skull) of 19 individuals to study the body plan of these 430k-year-old human remains. The fossils represented both children and adults and all bones of the human skeleton.

The team found that the fossils were significantly phylogenetically related to
Neanderthals based on the skeletal morphology. They show a “wide Homo” body plan, an attribute inherited and maintained from earlier hominid ancestors (modern day humans have a “narrow Homo” plan).

Despite their wide bodies, the fossils had an estimated stature (height) that can be found in modern middle-latitude human populations, a trait that first appeared 1.6–1.5 million years ago. The large femoral head (the ball on the femur that sits in the hip socket) represented in the skeletons is thought to be a more recent morphological trait, acquired in Europe and Africa between 100 and 600 thousand years ago.

Interestingly, the size variation of the bones show that this group of hominids likely had a level of interpopulation dimorphism (variation in appearance), similar to the differences found in modern day humans.

Though these early humans do share a few skeletal traits with us modern day humans, they had a brain mass smaller than the Neanderthals and had more similar cranial features to this extinct hominid lineage as well.

Based on their evidence, the researchers suggest that these early hominids found at the Pit of Bones are likely a sister group of the later Neanderthals.

However, many other traits are also shared between ancestor and future species, leading the group to conclude that their fossil collection provides evidence for mosaic evolution, or evolution of different body parts at different times and speeds.

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