For the study, researchers from the University of Cambridge analyzed the height and weight of 311 hominin specimens, the ancestral lineage of which Homo sapiens alone still exist, dating from 4.4 million years ago to the modern humans after the last ice age.
Results revealed that the height and weight of humans four million years ago evolved at different speeds. In addition, the research showed that hominin bodies evolved in “pulse and stasis” fluctuations, instead of continuously increasing in size. The overall results of the study suggested three key stages of the body evolution of different hominin species – there was an increase in both height and weight between 2.2 to 1.9 million years ago; an increase in height alone about 1.4 to 1.6 million years ago; and an increase in weight around 500,000 to 400,000 years ago.
“From then onwards, average body height and weight stays more or less the same in the hominin lineage, leading ultimately to ourselves,” Manuel Will of Cambridge's Department of Archaeology, lead author of the study, said.
In addition, he explained that a leaner body with long legs and narrow hips and shoulders would have been created if there was a sole increase in height.
“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,” he said.
He also theorized that the higher surface-to-volume ratio of a tall, lean body proved to be an advantage for hours of animal stalking in the dry heat because a larger skin area provided more capacity for sweat evaporation. In addition, he said that the later addition of body weight occurred at the same time with the rise of migrations into higher places. A bulkier body would be more suitable for thermoregulation in colder weather conditions in Eurasia. Despite these valid theories, Will noted that the large gaps in the fossil record still conceal absolute truths. The researchers often estimated body sizes from highly fragmented fossil fuels.
However, they also discovered that two other species, Homo naledi and Homo floresiensis, had different body sizes. Will said that they may either have had older small-bodied ancestors or adapted to evolutionary pressures in small and isolated populations. These evolutionary pressures, according to the researchers, include cladogenesis – the splitting of lineage, and sexual dimorphism.
“Our study shows that, other than these two species, hominins that appear after 1.4 million years ago are all larger than 140cm and 40kg. This doesn't change until human bodies diversify again in just the last few thousand years,” Will said.
According to an article by the Scientific American, 60 to 80 percent of the difference in height between individuals is attributed to genetics, while about 20 to 40 percent is determined by nutrition. Height heritability varies from one population to another, and even between genders. Meanwhile, different populations who come from the same genetic background might still have different heights based on environmental effects. This is why the BMI as a measurement of health, is more of a reference than a guideline.
On the other hand, the factors that affect body weight are personal factors such as age, gender, race, diet, and exercise, community factors, societal influences, and social norms. (Related: The domestication of humans: poor diet and exercise causing human evolution to favor early obesity.)
Find out more research on humans at Research.news.