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When did humans and chimpanzees split from a common ancestor?

The human and chimpanzee lineages diverged from a common ancestor roughly 6 to 8 million years ago — the range the Smithsonian's Human Origins Program gives, and the one supported by the oldest candidate hominin fossils such as Sahelanthropus tchadensis (~7–6 million years) and Orrorin tugenensis (~6 million years). Older molecular-clock work using a faster assumed mutation rate produced a younger figure of about 5 to 7 million years, while clocks calibrated on directly measured per-generation mutation rates and on wild-ape generation times push the estimate back to roughly 7 to 13 million years, which is why several different ranges circulate in reputable sources. Humans carry 23 pairs of chromosomes (46) while chimpanzees, gorillas and orangutans carry 24 pairs (48) because two ancestral ape chromosomes fused end-to-end to form human chromosome 2 — a fusion still visible as leftover telomere sequence and a second, inactivated centromere. The separate claim that a "cognitive revolution" transformed human minds about 70,000 years ago is Yuval Noah Harari's popular framing, not an archaeological consensus.

The date: 6–8 million years, not a single number

There is no exact year, and any source that gives one is overstating the evidence. Divergence dates come from two independent kinds of evidence that constrain each other.

Fossils give a minimum age. If a fossil already sits on the human side of the split, the split must be older. Sahelanthropus tchadensis, found in Chad in 2001 and described in 2002, is dated to roughly 7 to 6 million years ago. Orrorin tugenensis from Kenya dates to about 6.2 to 5.8 million years ago, and Ardipithecus kadabba to roughly 5.8–5.2 million years ago. Whether all three are truly hominins is debated, but they push the split toward the older end of the range.

Genetics gives a rate-dependent estimate. A molecular clock counts the genetic differences between two species and divides by an assumed mutation rate. Human and chimpanzee genomes differ by roughly 1.2% in single-nucleotide substitutions, as reported in the 2005 chimpanzee genome paper in Nature. Counting insertions and deletions as well adds a further 4 to 5% difference, a point the Smithsonian's own genetics page makes explicitly. With the faster mutation rate assumed in the 1990s and 2000s, that difference converts to about 5–7 million years. Direct sequencing of parent–child trios later measured a slower per-generation mutation rate, which pushes the same genetic distance further back — some analyses to roughly 7 to 13 million years ago (Scally & Durbin, Nature Reviews Genetics 2012; Langergraber et al., PNAS 2012). The Smithsonian's 6 to 8 million years is the sensible middle, compatible with both the fossils and the revised clock.

A further complication, and a contested one: the two lineages may not have separated cleanly. One influential model (Patterson et al., Nature 2006) proposed a period of incomplete separation with continued gene flow, but critics have argued the same signal is explained by ancestral population structure and by variation in mutation and recombination rates rather than by hybridisation. If that model holds, "the split" is a process spanning a long stretch of time, not an event on a calendar.

Why humans have 46 chromosomes and chimpanzees have 48

Humans have 23 pairs of chromosomes; chimpanzees, bonobos, gorillas and orangutans all have 24 pairs. The difference is not a missing chromosome — it is a merger.

Human chromosome 2 corresponds, band for band, to two separate chimpanzee chromosomes (labelled 2A and 2B). Chromosome-banding comparisons in the late 1970s and early 1980s made the correspondence visible, and DNA sequencing later confirmed the mechanism: two ancestral ape chromosomes fused telomere-to-telomere (end-to-end) in the human lineage.

Two traces of that event survive in the sequence. First, telomeres — the repetitive TTAGGG caps that normally sit only at chromosome ends — appear in a degenerate, head-to-head array in the middle of human chromosome 2, exactly where the join would be. This was reported by IJdo and colleagues in the Proceedings of the National Academy of Sciences in 1991. Second, human chromosome 2 carries a second, non-functional centromere remnant at the position matching the centromere of chimpanzee 2B.

The count difference therefore tells you nothing about how related the species are; chromosome fusions and fissions are common in mammals. It is instead one of the more direct pieces of evidence for common ancestry, because a fusion scar is exactly what shared descent predicts and exactly what independent origin does not.

The "cognitive revolution 70,000 years ago" is contested

The claim that human cognition was transformed roughly 70,000 years ago is best known from Yuval Noah Harari's Sapiens, where a "Cognitive Revolution" around that date gives Homo sapiens language, fiction and large-scale cooperation. The idea has an older academic ancestor — the "Upper Palaeolithic revolution" or "human revolution", usually placed nearer 50,000–40,000 years ago and based largely on the sudden appearance of cave art, bone tools and ornaments in Europe.

Many archaeologists reject the revolution framing. The influential 2000 review by Sally McBrearty and Alison Brooks, The Revolution That Wasn't, argued that the European "explosion" is largely an artefact of where archaeologists dug: in Africa, the diagnostic modern behaviours appear piecemeal across the Middle Stone Age, tens of thousands of years earlier. Supporting African evidence includes engraved ochre and shell beads from Blombos Cave in South Africa dated to roughly 100,000–70,000 years ago, an ochre drawing on stone from the same site dated to about 73,000 years ago, engraved ostrich eggshell from Diepkloof around 60,000 years ago, and pigment use at Pinnacle Point from about 164,000 years ago (Marean et al., Nature 2007).

What is reasonably well supported near 70,000–60,000 years ago is demographic, not neurological: the main out-of-Africa dispersal that founded most non-African populations falls in roughly that window. Conflating a dispersal with a cognitive mutation is where the popular story overreaches. Note also that 70,000 sits close to the Toba supereruption (~74,000 years ago); the once-popular "Toba bottleneck" explanation for a sudden cognitive leap is now widely doubted.

What most scholars accept, and what stays contested

Frequently asked questions

Did humans evolve from chimpanzees?

No. Humans did not evolve from chimpanzees; the two species descend from a common ancestor that lived roughly 6 to 8 million years ago and was neither a human nor a modern chimpanzee. Chimpanzees have been evolving for exactly as long as humans have since that split. The common ancestor's anatomy is not directly known, because no fossil has been securely identified as belonging to it.

Why do humans have 46 chromosomes when chimpanzees have 48?

Because two ancestral ape chromosomes fused end-to-end in the human lineage to form human chromosome 2, reducing 24 pairs to 23. The fusion left detectable scars: a degenerate, head-to-head array of telomere repeats in the middle of chromosome 2 — reported by IJdo and colleagues in PNAS in 1991 — and a second, inactive centromere remnant. Human chromosome 2 matches chimpanzee chromosomes 2A and 2B end to end.

Was there really a cognitive revolution 70,000 years ago?

The 70,000-year 'Cognitive Revolution' is Yuval Noah Harari's framing in Sapiens and is not an archaeological consensus. Many archaeologists, following McBrearty and Brooks's 2000 paper The Revolution That Wasn't, argue that symbolic and complex behaviour accumulated gradually across the African Middle Stone Age — engraved ochre and shell beads at Blombos Cave date to roughly 100,000–70,000 years ago, and pigment use at Pinnacle Point is older still. What does fall near 70,000–60,000 years ago is the main out-of-Africa dispersal, a demographic event rather than a cognitive one.

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