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Comment to the reader.
The previous entry ended in the early Eocene, with the small, large-eyed, tree-dwelling primate Teilhardina moving rapidly across the Northern Hemisphere during a brief warm spike at the boundary between the Paleocene and Eocene epochs. The basic primate body plan — forward-facing eyes, grasping hands, social groups, long childhoods — was in place. What had not happened, and would not happen for another fifty million years, was that any branch of the primates climbed back down out of the trees.
This entry follows what happened when one branch did. It covers the appearance of the earliest hominins, the move to habitual bipedal walking, the manufacture of stone tools, the appearance and expansion of the genus Homo, the use of fire, the brief mid-Pleistocene world in which several human species lived simultaneously, the appearance of Homo sapiens, and the spread of Homo sapiens across every continent of the planet by the end of the last ice age. The story closes about twelve thousand years ago, just before the agricultural revolution that begins the next major arc of the project.
I write in the spring of 2026 of the Common Era. Several of the dates and benchmarks given here have moved within the past decade, and a few have moved within the past two or three years. Where a working benchmark has shifted recently or remains contested, I have flagged it in the prose.
This entry is also the close of the How We Came About science series, which opened seven entries ago with the origin of life. The arc moves from the first self-replicating chemistry on the early Earth to the genome of every reader. The next series will pick up at the agricultural revolution.
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What had to happen, between an Eocene primate the size of a mouse lemur and the species writing these words, for an animal of our kind to appear on the planet?
The previous entry ended fifty million years ago in the canopy of broadleaved forests, with the early primates already split into the two main lineages they have followed ever since: the strepsirrhines, leading to the modern lemurs and lorises, and the haplorhines, leading to the tarsiers, the monkeys, the apes, and us. From here, the arc narrows further. One branch within the haplorhines will produce the great apes; one branch within the great apes will produce the hominins, the lineage of upright-walking forms more closely related to modern humans than to chimpanzees; one species within the hominins will spread across the planet, occupy every continent except Antarctica, and, as the last ice age ends, begin the long process of building what its descendants now call civilization.
This is the story of that narrowing.
The most recent common ancestor of human beings and chimpanzees lived in Africa somewhere between roughly six and eight million years ago. That estimate is anchored by two independent lines of evidence — the fossil record, which contains plausible early hominin candidates from around seven million years ago, and the molecular clock, which uses the rate at which neutral genetic differences accumulate between living species to estimate when their lineages last shared a common ancestor.
The molecular-clock estimate has shifted substantially over the past few decades. Early estimates based on a constant per-year mutation rate placed the chimp–human split around five to six million years ago. The shift came from direct measurement of the human germline mutation rate using parent–offspring whole-genome sequencing, and from more careful calibration against great ape generation times: the rate per year is slower than older work assumed, which pushes the split further back into the past. The 2026 working estimate, drawing on these calibrated rates, places the chimp–human last common ancestor around seven million years ago, with the genuine uncertainty spanning roughly six to nine million years [1].
The animal at that node was not a chimpanzee. It was a now-extinct great ape from which both lineages descend. After the split, the chimpanzee lineage continued to occupy the African forest and produced the modern chimpanzees and bonobos. The hominin lineage left a long, fragmentary, often contentious fossil trail across Africa over the next seven million years.
The fossils from the first two million years after the split are scarce and difficult to interpret. Three named genera carry most of the weight in the current literature.
Sahelanthropus tchadensis is known principally from a remarkably complete cranium, Toumaï, recovered from the Toros-Menalla locality in northern Chad and described by Brunet and colleagues in 2002. Sahelanthropus is dated to approximately seven million years ago, on a combination of biostratigraphy and the more recent radiometric and cosmogenic dating of the host rocks [2]. The cranium has a chimpanzee-sized braincase, a small face, and reduced canines, which would not in themselves make it a hominin. The principal argument for placing it on the human side of the split is the position of the foramen magnum, the opening in the base of the skull through which the spinal cord passes. In Sahelanthropus this opening is positioned beneath the skull rather than behind it, the position one would expect in an animal that habitually held its head upright over a vertical body. A femur attributed to the same individual was described in 2022 and was argued to support habitual bipedal locomotion, but the femur attribution and the bipedalism inference have both been contested in subsequent literature, and the question is not yet settled [2].
Orrorin tugenensis, from the Lukeino Formation of the Tugen Hills in Kenya, is dated to roughly six million years ago on current stratigraphic calibration of the host rocks and was described by Senut and colleagues in 2001 [3]. Orrorin is known from fragmentary remains, including a femur whose internal cortical thickness distribution is consistent with bipedal weight-bearing — a feature that, like Sahelanthropus's foramen magnum, is suggestive without being conclusive.
Ardipithecus kadabba (~5.8 to 5.2 Ma) and the much better-known Ardipithecus ramidus (~4.4 Ma) come from the Middle Awash region of Ethiopia. The 2009 Science monograph on A. ramidus, led by Tim White and a large international team, presented a partial skeleton named Ardi: a small-bodied animal with grasping feet still capable of climbing trees, but with a pelvis and lower limbs reorganised for upright walking on the ground [4]. Ardipithecus is widely accepted as a hominin, though the question of whether it sits on the direct human line or on a sister branch is open.
The pattern across these earliest candidates is consistent. Several lineages of African apes appear, between roughly seven and four million years ago, to have begun spending more time on the ground and walking upright when they did so. None of them was an obligate biped. The fossils are fragmentary, the dates are revised regularly, and the placement on or near the hominin trunk is contested in detail. What the record establishes firmly is that the move toward bipedalism began early — close to the chimp–human split itself — and that several lineages were experimenting with it.
By around four million years ago, the hominin record becomes denser, the bipedalism becomes unambiguous, and a recognisable adaptive radiation gets underway. The animals that fill this stretch are usually grouped as the australopithecines, after the genus Australopithecus.
Australopithecus anamensis, from rocks in northern Kenya and northern Ethiopia, dates to roughly 4.2 to 3.8 million years ago. A well-preserved cranium from Woranso-Mille in Ethiopia, described by Haile-Selassie and colleagues in 2019, gives the clearest picture of what the early australopithecines looked like — a chimpanzee-sized animal with a forward-projecting face and a small brain, but with the hip and knee structure of a habitual biped [5]. A. anamensis is the immediate predecessor of, and probably ancestral to, the much better-known Australopithecus afarensis.