A fossil analysis spanning more than four million years suggests that height and body mass advanced at different speeds throughout hominin evolution.
This new study is the largest study of hominin body sizes of all time. In it, an international team of researchers looked at 311 species that dated from earliest upright species of 4.4 million years ago all the way to the modern humans that followed the last ice age.
While the evolution of assorted hominin species typically shifted in many random ways, scientists say that there are some broad patterns in the data that show bursts of growth at key stages, followed by plateaus where almost nothing changed for many millennia.
For example, though hominins grew about 10 centimeters taller roughly one-and-a-half million years ago, they would not consistently get heavier for another couple million years.
That event is significant because, before that time, hominin height and weight likely stayed in concert with each other.
“An increase solely in stature would have created a leaner physique, with long legs and narrow hips and shoulders,” said lead author Manuel Will, a researcher from Cambridge’s Department of Archaeology, according to Phys.org. “This may have been an adaptation to new environments and endurance hunting, as early Homo species left the forests and moved on to more arid African savannahs,”
This pattern makes sense because a tall, slender body would have been advantageous when it came to hunting for hours in the dry heat. However, as humans moved into cooler areas, they needed to gain weight to better combat the cold.
The team found that body size was highly variable throughout early hominin history, with a range of differently shaped species, but it slowly shifted towards heavier body sizes over time.
The first big change occurred when the species Homo came about between 2.2 and 1.9 million years ago. At that time, hominins expanded in both height and weight. Then, only height increased right around the emergence of Homo erectus between 1.4 to 1.6 million years ago. Consistently heavier hominins did not come about in the fossil record until roughly a million years after that point, and both height and weight have stayed the same since then.
Those findings show that there were strong selective pressures against small body sizes which shifted the evolutionary spectrum towards the larger bodies of modern humans. That could have contributed to ‘cladogenesis’ — where a lineage splits — and caused smaller species to go extinct. In addition, sexual dimorphism — the physical distinction between genders — used to be more common in hominin species before being taken out by evolution.
This information gives a better link into the history of our ancestors and could provide more information on our own evolution moving forward.
“Body size is one of the most important determinants of the biology of every organism on the planet,” added Will. “Reconstructing the evolutionary history of body size has the potential to provide us with insights into the development of locomotion, brain complexity, feeding strategies, even social life.”
The new study is published in the journal Royal Society Open Science.