Early Humans Became Tall and Thin 1.5 Million Years Ago to Survive Outside the Forest

Femoral head bones of different species illustrating the size range in the hominin lineage. From top to bottom: Australopithecus afarensis (4 million to 3 million years; ~40 kg, 130 cm); Homo ergaster (1.9 million to 1.4 million years; 55-60 kg; ~165 cm); Neanderthal (200.000 to 30.000 years; ~70 kg; ~163 cm). University of Cambridge

For most of hominid evolution, our ancestors got heavier as they got taller. However, about 1.5 million years ago, humans had a growth spurt, suddenly becoming tall and lanky. This was likely a response to changes in human behavior.

In research published in Royal Society Open Science, scientists at Cambridge's Department of Archaeology discussed their analysis of 311 specimens of upright-walking hominids. What they found was that at one point, human ancestors gained about 4 inches in height, becoming taller and more slender than their predecessors.

"An increase solely in stature would have created a leaner physique, with long legs and narrow hips and shoulders," said lead author from Cambridge Manuel Will, 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 savannas."

The researchers looked at data from hominid bones as ancient as 4.4 million years ago and as recent as the Holocene, or modern times. They reported that most of our history included people with averages of relatively consistent body sizes, but that history was punctuated with a few considerable gains in height and weight. At one point, it was height that disproportionately changed.

Being stocky and strong has its benefits, but so does being tall and lean. Taller people can see farther over the savanna, and with long legs, they can run longer distances in pursuit of prey.

Ancient humans hunted animals, such as deer and antelope, that could often run much faster. However, humans had various other ways to catch prey. One is called "persistence hunting," where a person or animal will simply keep chasing their prey until it is exhausted. Taller, leaner, bipedal humans were more adapted to these long distance runs in grassland environments than their strong, wide family in the forest. In effect, our ability to run long distances was pivotal in our evolution.

Furthermore, a taller person has more surface area as opposed to volume, so they can sweat more and retain less heat. Having less hair than other animals means that humans can cool down by sweating, whereas other animals pant, and often have to slow down to do so.

A million years after the growth spurt—about 500,000 years ago—hominids gained another 20 to 30 pounds. Moving to a colder climate meant that they needed to put on some insulating fat.