Imagine the scene, around 3 million years ago in what is now east Africa. By the side of a river, an injured antelope keels over and draws its last breath. The carcass is soon set on by hyenas, who tussle with a crocodile. The crocodile surfaces and grabs part of the animal.
Author
- Tom O'Mahoney
Senior Lecturer in Biomedical Sciences, Anglia Ruskin University
The hyenas win and the crocodile retreats with only a leg. After having their fill, the hyenas slope off. Some funny-looking apes approach, walking upright. They have what appear to be stones with sharp edges in their hands. They hurriedly cut off some scraps of meat and start chewing at them.
Their squabbling attracts the attention of a nearby Homotherium (an extinct, scimitar-toothed big cat) who creeps up and suddenly breaks cover. Will these strange apes survive the encounter? Can they run fast enough, and far enough?
Our team's research modelled the anatomy of these early humans, Australopithecus afarensis , to find out how well they could run. Australopithecus afarensis is one of the best-known early human ancestors dating from 2.9-3.9 million years ago.
The partially complete Australopithecus afarensis skeleton Lucy , or Dinkʼinesh (Amharic: ድንቅ ነሽ, lit."you are marvellous") is globally iconic as a representation of early bipedalism (the ability to walk on two legs). Found in the Afar Depression in north east Ethiopia, this discovery received worldwide attention when it was made in 1974. It was evidence that brain expansion evolved after human ancestors started walking on two legs, as scientists had long believed.
Some researchers have also linked Australopithecine anatomy to an, as yet unknown, knuckle-walking common ancestor of humans, gorillas and chimpanzees. This hypothesis has since been refuted.
Scientists now believe that knuckle-walking probably evolved several times in apes, as the style of walking and internal architecture of ape hands and elbows are subtly different from each other. Researchers also think that the anatomy we see in hominins reflects an adaptation for upright movement in trees in a distant ancestor.
Early bipeds, such as Ardipithecus kadabba which looked a bit like a gorilla, lived in Africa between 5.8 and 5.2 million years ago. They lived in mosaic habits (a mixture of open and wooded landscapes) so some adaptation to moving in trees would make sense.
Until recently, scientists thought that only animals of the genus Homo, which emerged around 2 million years ago, made stone tools. The discovery of cut-marked bones in Dikika, Ethiopia (in 2009) dated at 3.4 million years, and in 2011 of stone tools at Lomekwi, Kenya from 3.3 million years ago , changed scientists' ideas of how much access Australopithecus had to meat.
The debate is now more a matter of whether Australopithecus regularly killed animals themselves, or if they were eating from carcasses after other predators (secondary access).
For primary access and regular kills, they needed to be able to do two things. Run fast (bursts of speed to outpace an unaware animal), and run for long amounts of time (to wear down a prey animal).
This is the endurance running hypothesis . The emergence of this behaviour is thought to coincide with more modern anatomy, such as seen in Homo erectus, who lived from around 2 million years ago to around 1 million years ago. The best way to test if Australopithecus was capable of endurance running at what we consider "modern" speeds is to reconstruct the skeleton of Australopithecus afarensis and simulate how they may have moved.
To try and answer this question, my team reconstructed the complete skeleton of Lucy, using 3D modelling. Where parts were missing, we estimated these using scaled versions of other Australopithecus skeletons. Since Lucy is a shared ancestor for chimpanzees as well, we also morphed Australopith and modern human and chimpanzee skeletal material, using an analytical technique called geometric morphometrics .
We then started putting muscles onto the bones of the pelvis and lower limbs of Australopithecus and a modern human model, using the open source software Gaitsym. Muscles and other soft tissues are not preserved in fossils so we varied the muscle properties from chimpanzee-like to human-like, producing a range of estimates for running speed and economy.
We also ran multiple simulations where we added and removed a long Achilles tendon, which chimpanzees don't have, as it is thought to affect running speed and energy use by enhancing recovery.
This was a team effort, with reconstructions across multiple labs. The simulations were run on the high performance computing facilities at the University of Liverpool.
These simulations revealed that Lucy wasn't as good at running as modern humans. The top speed our simulations could produce was 11mph, with a minimum of about 3.35mph. Elite sprinters, however, can reach peak speeds of more than 20mph. Even non-elite sprinters can reach around 17.6mph.
We also found that the metabolic cost of transport (how much energy it takes to move) was between 1.7 and 2.9 times higher in Lucy than in a modern human. The more "ape like" you make the muscle architecture and the shorter you make the Achilles tendon, the higher this cost is.
It appears that modern human limb proportions, combined with key changes in architecture of the calf muscle (such as relatively short fibres and large cross sectional areas), plus a long Achilles tendon, enabled much faster running in the genus Homo.
This means that it was probably not physiologically possible for Australopithecus afarensis to engage in persistence hunting, unlike later species of the genus Homo species.
Going back to our story at the start, it is likely the Australopithecines in this group wouldn't have escaped the big cat. They simply couldn't run fast enough, or for long enough.
Tom O'Mahoney does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
