Human Origins: How Did We Become Human?

We became human in stages that did not line up, and the order is the most important thing about human origins. Walking upright is the oldest part: a form of bipedality was present 4.4 million years ago, possibly earlier. Stone tools appear 3.3 million years ago, long before any large-brained hominin. Brain size reached the modern range about 300,000 years ago, but modern brain shape only between 100,000 and 35,000 years ago. Skin and hair left no fossils at all, so their dates are inference rather than measurement. And for most of our existence we shared the planet with other human species.

Five changes, five different clocks

The usual version of this story is a queue: stand up, grow a brain, make tools, leave Africa. The evidence shows five changes with five separate timetables, each measured by a different instrument — fossils and footprints for posture and brain size, excavated stone and ash for behaviour, comparative anatomy across primates for skin and hair, ancient DNA for contact between populations.

The instruments are not equally good, and that asymmetry shapes the whole account. Bone and stone can often be dated directly; skin cannot be dated at all. Any confident date for the loss of fur is filling a hole in the record rather than reporting from it.

Table 1 gives the anchors used below. Read it as dated specimens, not as a lineage.

Species or sample	Where	Age	Key features	Still disputed
Sahelanthropus tchadensis	Chad	6-7 Ma	Nearly complete cranium, small canines	Whether it was bipedal; almost no limb bones
Ardipithecus ramidus	Ethiopia	4.4 Ma	Partial skeleton; climbing plus primitive bipedality	How much time it spent on the ground
Early Homo (three lineages)	East Africa	2.5-1.5 Ma	Body and brain traits arrived separately	Where one species ends and the next begins
Homo from Dmanisi	Georgia	~1.8 Ma	Complete skull, 546 cm³ braincase, large face	How many species the sample represents
Homo sapiens from Jebel Irhoud	Morocco	315 ± 34 ka	Modern face and teeth, archaic braincase	Where in Africa the species began
Homo floresiensis	Flores	100-60 ka	Small-bodied island hominin	Which earlier population it descends from

Standing up came first

The oldest candidate is Sahelanthropus tchadensis, from Toros-Menalla in Chad, 2,500 km west of the East African Rift. The associated fauna puts the fossils at 6 to 7 million years old (Brunet et al. 2002). What was recovered is a nearly complete cranium plus jaw fragments and teeth: no pelvis, no foot.

A virtual reconstruction of the distorted cranium concluded that the skull base is consistent with an upright posture, so its authors suggested Sahelanthropus might have been a biped (Zollikofer et al. 2005). The operative word is "might": the claim rests on the angle of a reconstructed skull base, not on a limb bone. The earliest secure evidence of hominin posture therefore comes from later, and from lower down the skeleton.

Ardipithecus ramidus is that evidence, and it is far richer. More than 110 specimens from 4.4-million-year-old sediments in Ethiopia's Afar Rift include a partial skeleton with much of the skull, hands, feet, limbs and pelvis. The reconstruction combines arboreal palmigrade clambering and careful climbing with a terrestrial bipedality more primitive than Australopithecus, in what appears to have been woodland rather than grassland (White et al. 2009). The same paper draws a conclusion that complicates the popular picture: the last common ancestor we shared with chimpanzees probably differed substantially from any living ape. Chimpanzees are not a stand-in for that ancestor; they are a specialised lineage with as much evolutionary time behind them as we have.

By 3.6 million years ago walking was already efficient. The Laetoli footprints in Tanzania are the earliest direct evidence of hominin bipedalism, and an experimental comparison of footprint shape against humans walking with extended and with flexed limbs found the Laetoli trackmakers transferred weight most like the economical extended-limb gait of living people (Raichlen et al. 2010). Energetically human-like bipedalism predates the genus Homo by more than a million years.

Brains changed later, and in stages

Brain expansion belongs third or fourth here, not first. Traits once bundled together as "the origin of Homo" — large linear bodies, long hind limbs, big energy-expensive brains, reduced sexual dimorphism, more meat eating — did not arrive as a package. Integrated fossil, archaeological and environmental evidence shows some arising substantially earlier and some later than the old story had them, with three lineages of early Homo between about 2.5 and 1.5 million years ago (Antón, Potts & Aiello 2014). Whatever the genus was for at its origin, it was not a brain event.

Size also needs an explanation, because brain tissue is expensive. The expensive-tissue hypothesis argued that the human brain's cost, roughly a fifth to a quarter of resting metabolic rate, has to be paid for somewhere, and proposed a trade-off against gut size (Aiello & Wheeler 1995). Whether or not that exact trade-off holds, the accounting problem is real: a large brain is a budget item, not an automatic improvement.

The timing is now much sharper. Geometric morphometrics on endocranial casts from 20 Homo sapiens fossils shows that by 300,000 years ago brain size already fell inside the present-day range, while brain shape did not: the characteristic globularity evolved gradually within our own lineage, reaching present-day variation only between about 100,000 and 35,000 years ago, after the face had already become modern (Neubauer, Hublin & Gunz 2018). A modern-sized brain is thus older than a modern-shaped one. Endocranial volume is easy to measure and easy to over-read.

Tools and fire arrived before the big brains

In 2015 the assumption that the earliest stone tools were made by the genus Homo stopped working. At Lomekwi 3 in West Turkana, Kenya, in situ artefacts turned up in 3.3-million-year-old sediments, associated with Pliocene hominin fossils in a wooded palaeoenvironment, and the knappers combined core reduction with battering. The assemblage is 700,000 years older than the Oldowan, and its describers named it the Lomekwian (Harmand et al. 2015). Nobody knows which species made them, and no fossil of the genus Homo is known from that age: the toolmaker was a hominin we would not call human.

Fire is the same story with a longer argument attached. At Wonderwerk Cave in South Africa, micromorphology and infrared microspectroscopy of intact sediments produced burned bone and ashed plant remains in early Acheulean layers about 1.0 million years old — to its authors' knowledge, the earliest secure evidence of burning in an archaeological context (Berna et al. 2012). But one secure instance is not a habit. Reviewing the European record, Roebroeks and Villa conclude that early hominins moved into northern latitudes without habitual use of fire, that fire became a significant part of the technological repertoire only from roughly 300,000 to 400,000 years ago, and that claims of regular fire use in Africa around 1.6 million years ago are not uncontested (Roebroeks & Villa 2011).

Cooking turns the hearth into a hypothesis about anatomy. Wrangham and colleagues argued that cooking is the dietary adaptation separating humans from other apes, tying it to reduced tooth and gut size and to social life organised around a fire (Wrangham et al. 1999). The evidence stays indirect — no excavation reports "this fire was for cooking" — which is why it remains a hypothesis.

Bare skin, sweat and colour

Skin and hair do not fossilise, so this section has comparative anatomy instead of dates. Counting appendages in five skin regions of three primates — macaque, chimpanzee, human — shows the pattern is not simply "less hair". Human hair cover is greatly attenuated, but human follicle density is chimpanzee-like and well below a macaque's, while eccrine sweat gland density is on average tenfold higher in humans than in either, whose densities are strikingly similar. The authors conclude that reduced hair density came first, in the common ancestor of humans and apes, and that expanded eccrine glands plus the loss of fur cover happened only in our lineage (Kamberov et al. 2018). Fur loss and sweat-gland gain were not one event with one explanation; the glands are the uniquely human part.

Skin colour has a better quantitative account than either. Reflectance across indigenous populations correlates strongly with absolute latitude and with ultraviolet radiation, most strongly near 545 nanometres, close to the absorption maximum of oxyhaemoglobin, which points to melanin regulating UV effects on blood vessels in the dermis. The cline fits a compromise between photoprotection and vitamin D synthesis, with darker skin where UV is intense and lighter skin where it is scarce (Jablonski & Chaplin 2000). Colour is a UV adaptation and a clinical variable; it is not a taxonomy.

What none of this supplies is a date, and the number of times someone has confidently announced the age of human hairlessness measures how much people want the record to be more complete than it is.

Leaving Africa, again and again

The first dispersal did not need a big brain. Dmanisi in Georgia has produced five crania around 1.8 million years old, including a complete adult skull whose braincase measures 546 cubic centimetres — small, attached to a large prognathic face, and morphologically close to the earliest African Homo. The authors read the sample's wide variation as evidence for a single evolving lineage with phylogeographic continuity across continents (Lordkipanidze et al. 2013). Small-brained hominins were in Eurasia long before any handaxe-centred explanation of dispersal.

Our own species has a messier origin than a single birthplace. New fossils from Jebel Irhoud in Morocco combine modern facial, mandibular and dental morphology with more primitive neurocranial and endocranial morphology, and thermoluminescence dating of heated flints places them at 315,000 ± 34,000 years, with a uranium-series and electron-spin-resonance date on an associated tooth of 286,000 ± 32,000 years (Hublin et al. 2017; Richter et al. 2017). That makes Jebel Irhoud the oldest Homo sapiens yet reported, roughly 120,000 years older than the ~195,000-year-old eastern African fossil that had anchored the species' age, and it moves the story from a single origin to change across the whole continent.

Leaving that continent was not one event either. A maxilla and teeth from Misliya Cave in Israel date to between 177,000 and 194,000 years ago, making them the earliest modern human fossils outside Africa, well before the 90,000-to-120,000-year-old Skhul and Qafzeh material that used to hold the title (Hershkovitz et al. 2018). Early excursions plus a later, larger dispersal fit the record better than a single departure.

The other humans were neighbours, not ancestors

Neanderthals are known from genomes, not just bones. A draft sequence from three individuals showed that Neanderthals share more genetic variants with living people in Eurasia than with living people in sub-Saharan Africa, which means gene flow from Neanderthals into the ancestors of non-Africans before Eurasian groups separated from each other (Green et al. 2010). They are a sister lineage that interbred with us, not a rung below us.

Their end is dated rather than narrated. Improved radiocarbon work on 40 Mousterian and Neanderthal sites from Russia to Spain shows the Mousterian ending by 41,030-39,260 calibrated years before present across Europe, with the disappearance happening at different times in different regions and overlapping measurably with the earliest modern human industries (Higham et al. 2014). Coexistence was not a brief handover at the edge of a map.

Island species make the point differently. Homo floresiensis at Liang Bua was originally dated to between about 95,000 and 12,000 years ago, which would have kept a small-bodied hominin alive long after modern humans reached Australia; revised stratigraphy and chronology puts the skeletal remains at about 100,000 to 60,000 years old and the associated stone artefacts at about 190,000 to 50,000, leaving open whether the species survived past 50,000 years and met modern humans or Denisovans (Sutikna et al. 2016). Later discoveries have only added branches. "Human" was a crowded category for most of its history; our solitude is recent and unusual.

What can be dated, and what has to be inferred

Change	Earliest secure evidence	Kind of evidence	Main dispute
Habitual bipedality	4.4 Ma; human-like gait by 3.6 Ma	Skeleton, footprints	How early, and how much like us
Stone tools	3.3 Ma	Excavated artefacts	Which species made them
Brain expansion	Within early Homo, 2.5-1.5 Ma	Endocranial volume	Tempo, and what selection acted on
Fire	~1.0 Ma; habitual from 300-400 ka	Sediment micromorphology	Occasional use versus habit
Reduced fur, more sweat glands	Undated	Comparative anatomy	No direct clock exists
Modern brain shape	100-35 ka	Endocranial shape	What it implies for behaviour
Dispersal beyond Africa	~1.8 Ma	Fossils outside Africa	How many waves, and when
Interbreeding with archaic humans	Undated event; visible in genomes	Ancient DNA	Amount and regional pattern

The pattern is worth stating plainly: the changes with the best dates are the ones made of hard material, and the changes people care about most — skin, hair, behaviour, the moment of "becoming human" — have the weakest clocks.

Common misunderstandings

Evolution is a ladder with us at the top. It is a branching tree with one surviving branch. Early Homo alone contained three lineages between 2.5 and 1.5 million years ago (Antón et al. 2014), and at least two other human species were alive while our own was already widespread (Higham et al. 2014; Sutikna et al. 2016). Survival is not progress; it is what happened.

There is a "missing link" to be found. Transitional fossils are cousins, not ancestors. Sahelanthropus may sit near the base of the hominin branch, and even its describers could only say it might have been a biped (Zollikofer et al. 2005). The ancestor itself is a population we have not excavated.

We evolved from chimpanzees. We share an ancestor with them, and the 4.4-million-year-old Ardipithecus skeleton indicates that this ancestor was not chimpanzee-like; both lineages have since specialised in different directions (White et al. 2009).

Big brains came first, and tools built them. Stone tools are 700,000 years older than the Oldowan and predate the genus Homo (Harmand et al. 2015), while brain expansion belongs to the early Homo period and did not arrive as part of one adaptive package (Antón et al. 2014). The order is tools, then brains, then modern brain shape.

Fire was tamed early and kept ever after. There is secure burning at 1.0 million years (Berna et al. 2012), but habitual fire-keeping is a Middle Pleistocene habit from roughly 300,000-400,000 years ago (Roebroeks & Villa 2011). A single hearth is not a technology.

Skin colour sorts people into kinds. Reflectance tracks UV radiation as a cline produced by the competing needs for photoprotection and vitamin D synthesis (Jablonski & Chaplin 2000). It is one of the most visible human variables and one of the least taxonomic.

Frequently asked questions

Did humans evolve from chimpanzees?

No. Chimpanzees are our closest living relatives, not our ancestors, and they have had as much time to evolve since the split as we have. Ardipithecus ramidus, at 4.4 million years old, already shows a pelvis and foot unlike any living ape, so the shared ancestor was not a chimpanzee-like animal (White et al. 2009).

When did Homo sapiens first appear?

The oldest fossils assigned to the species come from Jebel Irhoud in Morocco, dated to 315,000 ± 34,000 years (Hublin et al. 2017; Richter et al. 2017). Those skulls combine modern faces with archaic braincases, and modern brain shape only appears between about 100,000 and 35,000 years ago (Neubauer et al. 2018), so the species boundary is a convention drawn across a long transition.

What came first, walking upright or big brains?

Walking upright. Ardipithecus ramidus walked bipedally at 4.4 million years ago (White et al. 2009), and the Laetoli footprints show human-like weight transfer at 3.6 million years ago (Raichlen et al. 2010). Stone tools appear at 3.3 million years ago (Harmand et al. 2015), and brain expansion follows the origin of the genus Homo (Antón et al. 2014).

Were Neanderthals our ancestors?

Mostly not. They were a sister lineage that interbred with the ancestors of living people outside Africa, so their DNA survives in us without their populations having been our direct ancestors (Green et al. 2010). The last of them disappear from the dated record around 40,000 years ago (Higham et al. 2014).

Why can we not date the loss of fur?

Because skin and hair do not fossilise. What can be measured is comparative: humans have chimpanzee-like follicle density but attenuated hair cover, and tenfold higher eccrine gland density than either chimpanzees or macaques (Kamberov et al. 2018). That establishes the order of the changes, not their dates.

Sources

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Explore this topic

This pillar is the hub for the Human Origins cluster. Two cluster articles are already published:

• They Buried Two Children With 13,000 Ivory Beads. Why?
• Why No Animal Has Ever Evolved a Wheel?

Two more are written and queued for publication, so they are listed by title only rather than linked:

• planned: Why Did Humans Lose Their Fur?
• planned: Was Oliver Really a Human-Ape Hybrid?

Everything carrying this pillar's label will collect at Human Origins as the cluster is published; until then that page falls back to unrelated posts.