For Intro to Anthropology, after exploring primate diversity, we read Chapter 5, “Human Origins & Earliest Hominins.” This page covers bipedalism and the earliest hominins. For stone tools and material culture, see Stone Tools and Human Evolution. Next is chapter 6 on human dispersal.
Key Concepts: Why Humans Evolved Bipedalism
- Bipedalism came first, big brains later. Habitual upright walking emerged around 7 million years ago; significant brain expansion didn’t follow for roughly 4 million more years (131). Our ancestors walked upright with chimp-sized brains.
- Walking upright was dangerous. Standing tall let small hominins see across the landscape, but it also let predators see them (135). For most of human evolution, our ancestors feared being hunted more than they hunted.
- The environment was patchy forest, not open savanna. African tropical forest was fragmenting into a mosaic of woodland and grassland (125), forcing creatures to move between habitat types.
- Multiple possible advantages, none proven. Bipedalism frees the hands to carry food and infants, aids thermoregulation by reducing sun exposure, and saves energy over long distances—but no single explanation is established.
- Endurance running, not speed. Humans can’t outsprint predators, but bipedalism plus sweating makes us uniquely capable of running prey down over distance (135).
- The “missing link” is a discarded idea. Scientists no longer look for links on a ladder; they look for common ancestors that resemble neither living descendant. The last common ancestor probably looked something between a gorilla and a chimpanzee.
- Bushy evolution, not a straight line. Many bipedal forms existed, lineages split and sometimes rejoined through hybridization, and traits appeared in a “mosaic” rather than a single package (136-137).
- Ardi (Ardipithecus ramidus, ~4.8-4.4 mya) has opposable thumbs and opposable toes—possibly a tree-adapted form of bipedalism (136).
- Lucy and the australopiths give secure skeletal evidence of habitual bipedalism, with hips and feet adapted for upright walking (138-140).
- The Laetoli footprints (3.6 mya, Tanzania) are hard evidence of a human-like gait preserved in datable volcanic ash (140-141).
📖 Book: The Human Story (Commissions earned)
Bipedalism First, Big Brains Later
In the old days, we believed human evolution was all about those big brains. Humans like to think we’re super smart, smarter than any creatures in the world. So we assumed that if we looked back in the evolutionary record, we’d find creatures with big brains—intelligent ancestors who gradually became us. We were looking for “missing links” and smooth transitions from ape-brained creatures to human geniuses.
We now know this is wrong. The first thing that happened wasn’t bigger brains—it was bipedalism. Around 7 million years ago, some creatures became habitually bipedal, walking around on two legs most of the time (131). We don’t see any significant increase in brain size for another 4 million years, until about 2.5-3 million years ago. So for millions of years, our ancestors were walking upright with brains no bigger than a chimpanzee’s.
This overturns a lot of assumptions. In the old model, you needed big brains first, and then you could get learning, tool use, and eventually culture. The biology had to change before the behavior could follow. We now think it’s much more intertwined—learning, tools, and culture co-evolve with biological changes over millions of years. Culture isn’t added on top of a finished biology; it transforms our developmental biology as we go.
You’re Going to Get Eaten by a Lion
In the old story, bipedalism was this glorious moment when big-headed creatures would pop up over the African savanna, look out across the landscape, maybe a spear in hand ready to hunt. But imagine the reality: these are creatures that we know skeletally to be about three or four feet tall—around as tall as preschoolers—popping their heads up over the African grasslands. Picture it: preschoolers with spears, striding out onto the savanna, ready to hunt.
What’s the first thing that’s going to happen once one of these creatures pops their head up? You’re going to get eaten by a freaking lion! A big cat is going to pounce, and that’ll be the last time you pop your head up over the grass.
As your textbook notes, yes, bipedalism may have enabled creatures to “see better across the landscape, but it also meant hominins could be better seen by predators” (135). For most of human evolution, our ancestors weren’t doing a lot of hunting. They were more afraid of being hunted. So why would bipedalism evolve at all?
Patchy Forests, Not Savanna
We used to think bipedalism developed on the great savannas or grasslands of Africa. We now know the environment was actually a patchy forest area, as the African tropical forest fragmented into more open areas (125). You had areas of forest and areas of grassland developing together. Creatures were differently adapted to these areas and often had to move between them to get food and resources.
This matters because it changes how we understand the pressures driving bipedalism. It wasn’t about striding across open plains. It was about navigating a complex, changing landscape.
Why Walk Upright?
Several factors may have contributed to the evolution of bipedalism, though none has been proven and there may have been multiple advantages at once:
Bipedalism frees the hands and those opposable thumbs we share with other large primates like bonobos and chimpanzees. Free hands can carry food and babies across a landscape—useful when you need to move between patchy forest areas.
There’s also thermoregulation. If your back is exposed to the sun, that’s a lot more surface area than simply having your head exposed. Standing upright may have been a way of cooling off the body in the African heat.
Most importantly, if you’re going to walk over distances and get resources from various areas, it’s more efficient to do this on two feet. It’s faster in some ways to be quadrupedal and move on all fours, but if you’re doing this over a sustained amount of time, bipedalism saves energy over long distances.
Later emerges endurance running (135). We are actually the only primate that can run a marathon without taking breaks. Some people say long-distance running is the one sport where humans have an advantage over other creatures. All other animals can jump higher proportionate to their bodies and run faster proportionate to their bodies, but because of bipedalism and the way we sweat, humans can do endurance running over time. Some researchers believe this was eventually important for how humans would hunt—not by being faster than prey but by tracking animals over long distances and eventually running them down.
There are also real disadvantages. Bipedalism brings chronic back problems later in life—a reminder that evolution involves trade-offs, not perfect design.
Ardi: Walking in Trees?
One of the coolest fossils we have from this period is Ardipithecus ramidus, nicknamed Ardi, dating to around 4.8-4.4 million years ago (136). It’s a fairly complete specimen compared to what we usually find—roughly 60% of the skeleton, painstakingly reassembled from many fragments. But Ardi is a funny creature. It has opposable thumbs, but it also has opposable toes, so it seems somewhat bipedal but in a confusing way.
Was Ardi walking in trees? (137) This might seem weird, but we do see primates today who use all fours while walking along branches. It’s possible this represents a different way of being bipedal—not the striding gait we associate with humans, but something adapted to moving through trees while upright.
The people who discovered Ardi and spent years reconstructing the skeleton were convinced it belongs on the main line of human evolution. Others debate this, suggesting it might just be another example of the many different creatures evolving during this period. Either way, Ardi shows this “mosaic of traits” (136-137)—some things that look familiar to us, and other things that look completely unfamiliar.
Lucy and the Australopiths
We have much more fossil evidence from the australopiths, who appear around 4.2 million years ago and persist until about 2 million years ago (138). One variety, Australopithecus afarensis (3.9-3.3 million years ago), gives us the skeleton known as Dinknesh or Lucy (139-140). It’s a quite complete skeleton of a bipedal, probably human ancestor.
The name “australopith” simply means “southern ape”—australo for southern, pithecus for ape—though all these finds come from Africa, many from what is now Ethiopia. Lucy was named in the 1970s after “Lucy in the Sky with Diamonds,” which the team was playing when they found her. She has since been renamed Dinknesh, an Ethiopian word meaning “you are marvelous”—part of a broader effort to restore proper names to finds long known only by Western nicknames.
With the australopiths, the skeletal evidence for bipedalism is much more secure. Their hip alignment is better adapted for upright walking. You don’t see the opposable toe as much—the toe is in line with the heel, more like our own feet.
The Laetoli Footprints: Hard Evidence
We also have a wonderful set of footprints preserved in volcanic ash at the Laetoli site in Tanzania, dating to 3.6 million years ago. This is a great find because we have not only skeletal evidence but actual footprints recorded in volcanic ash. And volcanic ash can be dated precisely based on when the volcano exploded.
It’s often hard to tell from anatomy alone exactly how creatures moved. But these footprints give us “hard” evidence of habitual bipedalism (140-141). We have creatures with a human-like gait—the big toe aligned and pushing off, two individuals of different sizes, perhaps a parent and child—walking across this landscape 3.6 million years ago. The australopiths are cool—it’s wonderful to have so much evidence when there was so little from earlier periods.
How Many Bipedal Species?
Here’s a trick question: how many bipedal species were there? The answer is lots. Or rather, we don’t know how many, partly because it’s hard to say whether these were actually separate species or variation within a species. This is where paleoanthropologists divide into “splitters,” who read each new fossil as a new species, and “lumpers,” who treat the same finds as variation within species we already know. The disagreement is genuine because the biological definition of a species—populations that can reproduce with one another—can’t be tested on bones. We can only estimate from how different the fossils look, and ancient DNA, where we have it, has often made the picture messier rather than cleaner.
Instead of thinking about one smooth line or single track of evolution, we now talk about “bushy evolution”—not one branch but a bush with many twigs. We talk about hybridization, where creatures might drift apart for a while then come back together and recombine genetically. And we talk about this “mosaic of traits,” where different characteristics arise at different times and combine in unexpected ways. Homo naledi, a later South African find with surprisingly sophisticated hands but a small braincase, is a striking example: the traits we expect to travel together simply don’t.
The human origin story is very complicated. There are many branches and mysteries. A lot depends on a very small amount of fossil evidence—sometimes we’re trying to figure out what a creature was like based on a single tooth (126). Most creatures spend their lives trying not to become fossils—trying to live, not die, not get stuck in tar pits. And when creatures die, they usually get eaten, not fossilized.
Paranthropus and “Extinction of the Fittest”
Among the australopiths, debate continues over whether they split into several species or represent variation within fewer. One robust group—larger and heavier-jawed than the gracile australopiths—is now placed in its own genus, Paranthropus. They had a remarkable run, from roughly 2.7 million years ago until about 1 million years ago: nearly 1.7 million years, far longer than Homo sapiens has yet existed.
Paranthropus seems to have been very well adapted to specific food sources in its forest environment. And that may be exactly why it disappeared. We talk casually about “survival of the fittest,” but evolution often delivers something closer to extinction of the fittest: a creature so finely tuned to one environment becomes vulnerable when that environment changes. As the forests shifted, the specialists died out. It’s a useful corrective to the progress narrative—we tend to study only the lineages that reached the present, forgetting how many highly successful creatures had long, stable runs and then ended. Whether our own species matches Paranthropus for longevity remains very much an open question.
Frequently Asked Questions
Did bipedalism or big brains come first in human evolution?
Bipedalism came first, by a wide margin. Habitual upright walking appeared around 7 million years ago, while significant brain expansion didn’t begin until roughly 2.5-3 million years ago. Our ancestors walked upright with chimpanzee-sized brains for millions of years, overturning the old assumption that intelligence drove human evolution (131).
Why did humans evolve to walk on two legs?
There is no single proven reason, and there may have been several advantages at once. Walking upright frees the hands to carry food and infants, reduces the body surface exposed to direct sun (aiding thermoregulation), and conserves energy over long distances. It later enabled endurance running. Each came with trade-offs, including greater visibility to predators and chronic back problems (135).
What is the “missing link,” and why do anthropologists reject the term?
The “missing link” comes from an outdated idea of evolution as a ladder of ascending forms. Anthropologists instead look for common ancestors, which need not resemble any living species—the last common ancestor of humans and chimpanzees probably looked something between a gorilla and a chimpanzee. Evolution is “bushy,” with many branching lineages, not a single chain.
What is the oldest evidence of human bipedalism?
Skeletal evidence for habitual bipedalism becomes secure with the australopiths, and the Laetoli footprints in Tanzania—dated to 3.6 million years ago through volcanic ash—offer hard physical proof of a human-like gait. Earlier fossils like Ardipithecus ramidus (~4.8-4.4 mya) suggest a more ambiguous, possibly tree-adapted form of upright walking (136-141).
Who was Lucy, and why is she also called Dinknesh?
Lucy is a remarkably complete Australopithecus afarensis skeleton (3.9-3.3 mya) found in Ethiopia in the 1970s and named after the Beatles song playing at the camp. She has since been renamed Dinknesh, an Ethiopian word meaning “you are marvelous,” reflecting a broader move to restore proper names to African finds (139-140).
Continue Your Journey
For stone tools and how material culture co-evolved with biology, see Stone Tools and Human Evolution. For primate diversity and why there’s no single “primate pattern,” return to Primate Diversity and Evolution. Explore the Human Origins Hub for more on evolution and anthropology.
For an advanced treatment of how walking itself is a learned skill that shapes our developing bodies—challenging the assumption that bipedalism is simply “biological equipment”—see Tim Ingold on why walking and cycling are both skills.