An international team of researchers has created the first 3D reconstruction of the ribcage of the Turkana Boy, a skeleton of the juvenile Homo erectus — the most complete skeleton of this hominin ever found — from Nariokotome, Kenya, and compared it to those of Homo sapiens and Neanderthal. Their results, published in the journal Nature Ecology and Evolution, show that the Turkana Boy had a deeper, wider and shorter ribcage than seen in Homo sapiens, suggesting Homo erectus had a stocky body shape, despite being considered the first long distance runner among our ancestors.
This is an artist’s reconstruction of Homo erectus. Image credit: Yale University.
Anatomically modern humans have a relatively tall, slender body shape that contrasts with, for example, the shorter, stocky, heavy bodied Neanderthals.
Paleoanthropologists have long assumed that human body shape originated with the first representatives of Homo erectus in the context of climate changes and the receding forests in tropical Africa, over two million years ago.
Our tall and slender bodies seem evolutionarily advantageous in the expanding hot and dry savannah, helping to avoid overheating and well suited to bipedal running over long distances in more open terrain.
Fossils attributed to Homo erectus point to longer legs and shorter arms than our earlier ancestors, the australopiths, which were bipeds when on the ground, but still retained some commitments to life in the trees.
Several modern body characteristics are particularly clear in the 1.5-million-year-old remains of the Turkana Boy, also known as KNM-WT 15000.
Studies of how this individual walked and ran had largely been restricted to the skeleton’s legs and pelvis. However, for endurance running its breathing capabilities would have been relevant as well.
But this aspect has not before been investigated in any detail because assessing the chest and breathing motion based on a jumble of rib and vertebra fossils is difficult with conventional methods.
However, with the introduction of increasingly sophisticated imaging and reconstruction techniques in recent years this type of examination is now possible.
“It appears that the fully modern human body shape evolved more recently than previously thought, rather than as early as two million years ago when Homo erectus first emerged,” said senior author Professor Fred Spoor, a researcher in the Centre for Human Evolution Research at the Natural History Museum, London, the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology, and the Department of Anthropology at University College London.
In the study, Professor Spoor and colleagues virtually reconstructed the ribcage of the Turkana Boy and compared it with that of modern humans and a Neanderthal.
“The results are now changing our understanding of Homo erectus. Its thorax was much wider and more voluminous than that of most people living today,” said Dr. Markus Bastir, a scientist in the Departamento de Paleobiología at the Museo Nacional de Ciencias Naturales, Madrid.
“Actually, the ribcage of Homo erectus seems more similar to that of more stocky human relatives such as Neanderthals, who would have inherited that shape from Homo erectus,” said co-author Dr. Daniel García Martínez, a researcher in the Departamento de Paleobiología at the Museo Nacional de Ciencias Naturales, Madrid, and the Centro Nacional de Investigación sobre la Evolución Humana.
The authors speculate that these changes to human body shape may have optimized breathing capabilities for long-distance running and other endurance activities.
“That Homo erectus was perhaps not the lean, athletic long-distance runner we imagined is consistent with more recent fossil finds and larger body weight estimates than previously obtained; this iconic ancestor was probably a little less like us than we portrayed over the years,” Professor Spoor said.
“Our own body shape with its flat, tall chest, and narrow pelvis and ribcage likely appeared only recently in human evolution with our species, Homo sapiens,” said co-author Dr. Scott Williams, a scientist in the Department of Anthropology at New York University.
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M. Bastir et al. Rib cage anatomy in Homo erectus suggests a recent evolutionary origin of modern human body shape. Nat Ecol Evol, published online July 6, 2020; doi: 10.1038/s41559-020-1240-4
