"Light will be thrown on the origin of man and his history."
- Charles Darwin (1809-1882)
"Most of what we know about human evolution comes from these: the fossilized bones of our ancestors. With their help, we've traced our evolution from small furry creatures to the big-brained beings we've become today. But bones can't tell us everything."
- Ziya Tong (1980-)
Source: https://news.janegoodall.org/
What makes us human? It’s a combination of things, clearly. Large brains, an ability to reflect on ourselves and our lives through our consciousness, and the capacities to build complex technologies and institutions. Throw in our facility with language enabling us to become accomplished communicators within and across sophisticated cultures, and our ability with higher order math and crisp decision-making which allow us to resolve problems galore.
Fair enough. We’re good.
But what was the trigger for us to diverge from the other primates, our close relatives, the early chimpanzees and great apes? Many would point to that brain of ours and the sophisticated thinking this facilitates.
But I’d give the award to something much more taken-for-granted: kind of hidden in plain sight. It’s got a range of technical names, including bipedal locomotion, perambulation, and non-brachiation. We all know it by its common name: walking upright.
There are many twists and turns in the story of the evolution of our ancestors leading to Homo sapiens. It is striking to observe changes in skull size and configuration, and skeletal structure and function, through the evolution of hominid species. Trace these through the Ardipithecus group (4-6 million years ago); the Australopithecus group (1-4 million); the Paranthropus group (2 million-600,000 years ago); and the Homo group (2.4 million–now). How and when our brain size increased, by what mechanisms we began to walk vertically compared to knuckle-walking or moving on all fours, and the nature of the increasingly complex cognitive wiring that supports our ability to talk, are amongst the intriguing questions.
In particular, the earliest paleontologists long wondered whether our growth in brain size came before or after our move to being bipedal. There were theories, of course, and some pitched academic battles amongst the giants of the field, but with the paucity of fossilized pelvic and lower limb remains, this had not been definitively clarified.
Consider the logic, and see what you think. Go back three and a half million years ago. A predecessor species of ours, Australopithecus afarensis, is a hunter-gatherer-forager on the African savannah. Does its brain size expand, confirming that the species gets smarter at exploiting the environment relative to other species, foreshadowing eventual success? With a bigger, smarter brain our ancestors would figure out there’s a lot more to take advantage of across the landscape to increase their range and food prospects. Quite simply, with more brains they become more competitive. Moving out of the trees then becomes an obvious strategy. Evolution into bipedalism could quite naturally occur as a consequence of roaming around the savannah, and this would then in turn free their hands to do other things, including making stone tools to help with cutting up meat, chopping foodstuffs, and eventually, to make spears and arrow points.
Source: https://www.history.com/news/denisovans-interbreeding-discovery
If you haven’t studied prehistoric archaeology or paleoanthropology, a plausible alternative is equally logical. In this scenario bipedalism precedes increased brain size, and comes early to Australopithecus afarensis. Mostly a tree-dweller, tentatively at first, the species spends increasing time on the forest floor. Afarensis can then live flexibly, shifting from the trees and onto the ground depending on needs. Over the millennia this ability creates increasing possibilities to expand the community and take advantage of more wide-ranging food possibilities. This in turn provides a richer natural and social environment. These are just the kind of fertile social opportunities that would as a result be stimulatory for increased brain size over time.
When phrased like that, it’s a delicately poised chicken-and-egg conundrum. For many decades, most people who thought about this, including professionals in the field, voted for the first option. Increased brain size leads to physical changes, creating a shift from brachiating amongst the trees to knuckle walking to upright walking. Rational, persuasive, and sound logic. Then some startling evidence was laid down.
One day 3.6 million years ago a volcano known as Sadiman erupted, at a place we now call Laetoli, in modern day Tanzania’s Eastern Rift Valley in Africa. It spewed ash across the landscape. A little later, it rained, cooling everything down. Good fortune then intervened. The ground became not too squishy such that something heavy would sink far into it, and not too hard that there would be no trail detectable if a creature traversed it. We can imagine it to be the texture of wet clay. When Sadiman erupted again, ash buried all the marks made by anything that had moved across the landscape, preserving them for millions of years.
Many animals etched their tracks on the Laetoli terrain, including hares, guinea fowl, rhinoceros, giraffes, elephants, and baboons. And some hominids—pre-humans—crossed the ground, too. They left an 88 feet (27 meters) long trail of just 70 footprints that gave us a glimpse back across those 3.6 million years to the first recorded walking.
These steps are up there with Neil Armstrong’s walk-around after he jumped off that ladder, as the first man on the moon. These are the two most important strolls anyone, anywhere, ever took.
The Laetoli footprints.
The footprints were discovered by a team of archaeologists led by the famed paleontologist Mary Leakey in 1978. They have revealed much more than might be thought at first glance about this line of our prehistoric forbearers.
The closeness of the footmarks, in two parallel tracks, reveals the Laetoli Australopithecus afarensis had short legs like their relative, the famous Lucy, whose fossilized remains comprised bone pieces making up 40% of what is a female Australopithecus afarensis. She was found by Don Johannsen in 1974 in the Hadar area of nearby Ethiopia. These creatures were 1.2 to 1.4 meters (3 feet 10 inches to four feet seven inches) tall.
The footprints have revealed that the species’ big toes, like ours, are in line with their foot, unlike an ape, which has divergent, thumb-like big toes helpful for climbing. They also, in contrast to an ape, had an arched foot. Like us, they walked heel first, and pushed off with their toe. It’s a type of foot-rocking motion with which we are all familiar whenever we watch someone perambulate.
Science often extracts important information from what is missing. The fact that there are no knuckle imprints nearby indicates that hominids were at this point in the habit of—that is to say, evolved for—walking fully upright. Recent studies of the footprints along with nearby fossil skeletons have found they walked with a human gait at a pace estimated to be much the same as humans.
Stratigraphic analysis completed in 1999 confirmed that the climate at the time the footprints were made was moister than now. Bipedalism and standing upright has been linked with a shift from forest to grassland due to climate change, suggesting perhaps that this change in weather was one of the initiators for the evolution of bipedalism in these hominids.
We’ve inferred that these hominids walked, how they carried themselves, what pace they moved at, and the climate they were in. But what about their relationship to one another? The pairs of footprints are just 12 inches, or a third of a meter, apart.
Were they walking together, side-by-side? Were they related? Are these the footprints of a prehistoric family going down to the local friendly watering hole, or visiting another family or hunter-gatherer band?
While it appears at first glance that there are only two tracks, science now reckons there were in fact at least three individuals, one of whom was walking in the larger one’s footprints. In February 2011, anthropologists Breithaupt and Musiba, from the University of Colorado at Denver, used a new photographic technique to create a three dimensional view of the footprints. They revealed an extra set of toes within one of the footprints, suggesting that one was tracking the other, perhaps playing follow-the-leader. They also found that these two footprints, one inside the other, had the same sized feet.
The researchers admit that it was initially tempting to suggest a nuclear family: “We kind of idealized the interpretation of the evidence to have a family, a mother, father and a youngster,” one of the researchers involved, told LiveScience. In the minds-eye of the researchers, this was mum and dad walking side by side with their youngster trailing behind, in dad’s footsteps. Anyone with a teenager will get the point and be sympathetic with that interpretation.
However, based on the new evidence, the fact that the feet are the same size, suggests a group of similarly aged hominids (http://www.livescience.com/16894-human-ancestor-laetoli-footprints-family.html). Some of this is, of course, speculation. What happened 3.6 million years ago during a stroll after some lava-flow had cooled is not possible to untangle, in reality.
On the scale of archaeological projects, a few footprints in east Africa, preserved over millions of years, is a small find. But what those steps have told us about these hominoids is of huge pre-historic significance. If the trillions of journeys large and small that we and our ancestors have made, none of have been anything like as important in understanding the precursors to us. Australopithecines did indeed walk upright before the critical cranial developments. Lucy’s bones, too, suggest bipedalism, and her braincase was very small.
Lucy and the Laetoli footprints viewed together show that bipedalism evolved before the brain expansion; but it was not till the footprints were discovered that this could be confirmed. Since then there have been other finds in the nearby regions of Turwel and Maka, among other places, which have made the Australopithecus afarensis the better known and understood of all early hominids (http://archaeologyinfo.com/australopithecus-afarensis/). Today we think that, as Mary Leakey suspected, the evolutionary step-change to walking freed the hands of the predecessor of Australopithecus afarensis for carrying things, and eventually, tool making and tool use.
The skull size of the Afarensis would have made them pretty bad company for a Homo sapien; not much more interesting than a modern-day chimp in fact. Although there are suggestions of stone tools not long after Lucy and Laetoli, at 3.4 million years ago, cave painting and other indications of an intellectually-rich life do not occur until much later. The first date for cave painting’s appearance, for example, is much later—35,000 years ago, at Maros on Sulawesi island, Indonesia. However Australopithecus afarensis hominids lived, we are drawn to conclude, they were not members of a culturally-sophisticated society (http://h2g2.com/edited_entry/A944336).
Humans are the best thinkers on the planet, and they can also hunt, fish, forage and farm pretty well. And as for dancing, painting, doctoring, cooking, writing, manipulating the environment, and inventing new tools, all become possible with freed-up hands, and so these skills—indeed, almost everything we do— owe a huge debt to bipedalism. So, then, do you.
Further reading:
Hay, Richard, Leakey, Mary (1982). Fossil footprints of Laetoli. Scientific American February: 50-57.
Musiba, Charles M (1999). Laetoli Pliocene Paleoecology: a Reanalysis via Morphological and Behavioral Approaches. PhD thesis: University of Chicago.
Pattison, Kermit (2020).Fossil men: the quest for the oldest skeleton and the origins of humankind. New York: William Morrow/HarperCollins.
Raichlen, David A, Gordon, Adam D, Harcourt-Smith, William EH, Foster, Adam D, Randall Haas, William Jr. (2010). Laetoli footprints preserve earliest direct evidence of human-like bipedal biomechanics. PLoS ONE 5 (3): e9769.
Video: Human Origins 101 | National Geographic (3:47)