Why Humans Have 46 Chromosomes but Chimpanzees Have 48

Why humans have 46 chromosomes while chimpanzees have 48 sounds, at first, as though humans simply lost two chromosomes. But that is not what happened. Two chromosomes in our ancestral lineage became joined into one — the chromosome we now call human chromosome 2.

Humans normally have 23 pairs of chromosomes, giving 46 chromosomes in most cells. Chimpanzees have 24 pairs, or 48. When their genomes are compared, almost every human chromosome has a clear one-to-one counterpart in the chimpanzee genome.

Human chromosome 2 is the striking exception.

Its sequence corresponds to two separate chromosomes in chimpanzees and other great apes. That relationship explains the difference in chromosome number.

When two chromosomes became one

A chromosome is not an indivisible unit of genetic information. It is a very long DNA molecule packaged with proteins into a physical structure.

Over evolutionary time, chromosomes can undergo major rearrangements. Sections may be inverted, duplicated, moved or deleted. Entire chromosomes can also fuse.

That happened in the lineage leading to humans.

Two ancestral chromosomes that existed separately became incorporated into a single chromosome. As the fused form eventually became established, two ancestral chromosome pairs were effectively represented by one pair of human chromosome 2. The diploid chromosome count therefore fell from 48 to 46.

The evidence is written directly into the genome.

The DNA sequence along human chromosome 2 corresponds, in order, to the sequences carried on two separate great-ape chromosomes. Where one ape chromosome ends and the next begins, the human sequence continues as a single chromosome.

Human chromosome 2 also contains molecular remnants associated with the ancestral junction and evidence of a second ancestral centromeric region.

A centromere is the specialized part of a chromosome that interacts with the machinery that separates chromosomes during cell division. A chromosome produced by the joining of two ancestral chromosomes would initially inherit centromeric regions from both. In modern human chromosome 2, one centromere remains functional while the other ancestral region has degenerated and lost that role.

The chromosome therefore acts almost like a piece of molecular archaeology: its present structure preserves evidence of how it was assembled in the past.

The fusion was not a simple piece of molecular glue

The classic picture of human chromosome 2 is straightforward: the ends of two ancestral chromosomes joined together.

The fusion itself is well established, but complete modern great-ape genome assemblies reveal that the history of the junction is considerably more complicated.

A 2026 study in Cell Genomics compared high-quality genomes from humans, chimpanzees, bonobos, gorillas and orangutans. The researchers found that the chromosome 2 fusion region carries evidence of segmental duplications, inversions and extensive turnover of repetitive DNA.

Segmental duplications are large pieces of DNA that have been copied to multiple locations in a genome. Regions rich in such repeated sequences are particularly prone to structural rearrangement.

Human chromosome 2 therefore did not arise from the perfectly clean joining of two otherwise unchanged chromosome ends. The surrounding genomic region had a complicated evolutionary history before, during and after the fusion.

The researchers placed the evolution of this complex fusion region roughly 5–7 million years ago, around the period when the lineages leading to humans and African great apes were separating. Reconstructing the precise sequence of events remains difficult because repetitive DNA and duplications obscure parts of that ancient history.

Why did the fusion not destroy the genome?

Joining two chromosomes sounds like a catastrophic mutation. Chromosomal rearrangements can indeed cause serious biological problems. But chromosome number itself does not measure the complexity of an organism, nor does one chromosome necessarily correspond to one unique set of genes that cannot be rearranged.

A fusion can preserve most of the genetic information from the two original chromosomes while changing the way that information is physically packaged.

The first carrier of the ancestral fusion probably had both the fused chromosome and the corresponding unfused chromosomes. Such an individual would have had a chromosome count different from either the ancestral 48-chromosome condition or the later 46-chromosome condition.

If the rearrangement remained compatible with development and reproduction, it could be transmitted to descendants. Over many generations, the fused form could spread through the population and eventually become fixed.

That does not mean there was a single individual who became the “first human” when the fusion occurred.

Species do not normally appear in one mutation. Human evolution involved populations accumulating many genetic and anatomical differences over very long periods of time.

There is also no evidence that chromosome 2 fusion alone created human intelligence or any other defining human trait. The 2026 study found that experimental deletion of parts of the fusion region in human neural progenitor cells altered the expression of some genes. That suggests the region can have regulatory effects, but it does not show that the ancient fusion caused the evolution of the human brain or created the reproductive separation between humans and other apes.

The simpler conclusion is already remarkable.

Humans and chimpanzees have different chromosome numbers not because one species is missing an enormous amount of genetic information. Much of the corresponding ancestral DNA has simply been packaged differently.

Millions of years later, the signature of that rearrangement can still be read in human chromosome 2.