Chimpanzees are our closest living relatives, and their DNA is strikingly close to ours. How close depends on what you count, so honest figures run from about 99% down to about 95%. The gap that makes us human lies less in how many DNA letters differ than in which ones differ and what they control.
Where the 98 to 99 Percent Figure Comes From
DNA is a long code written in four chemical letters, and each human cell carries about 3 billion of them. To compare us with chimps, researchers line up the two genomes the way you would lay two long texts side by side, then mark every spot where the letters don’t match. That only works for stretches that clearly correspond. Pieces with no counterpart in the other species are left out of the count, which is why this method gives the highest similarity figure of any.
When scientists line up matching stretches of human and chimp DNA, about 1.24% of letters differ in a sample of 1.9 million letters.1PubMed Central. Genomewide comparison of DNA sequences between humans and chimpanzees A larger comparison of 2.3 million letters found almost the same difference, 1.22%.2PubMed. Genomic divergence between human and chimpanzee estimated from large-scale alignments of genomic sequences In DNA that can be matched, roughly 99 letters out of 100 are the same. Reading it as “we are 99% chimp” confuses matching letters with how much of an animal is the same.
Even a 1.2% difference across 3 billion letters adds up to tens of millions of changes. The full chimp genome showed about 35 million single-letter changes since our lineages split.3PubMed. Initial sequence of the chimpanzee genome and comparison with the human genome Most of those sit in DNA that doesn’t code for proteins, and many have no known effect, so the count alone doesn’t say how many matter. The average also hides a lot, since some parts of the genome change far more than others.
Why Some Scientists Say 95 Percent or Less
Some differences aren’t swapped letters at all. They are whole stretches of DNA present in one genome and absent in the other, known as insertions and deletions. These range from a single letter to thousands, and they can arise when DNA is copied incorrectly or when pieces of DNA copy themselves into new spots. Because they change length rather than swap letters, methods that only compare matching positions missed or undercounted them.
In one 779,000-letter sample, letter swaps made a 1.4% difference and inserted or missing stretches added another 3.4%.4PubMed Central. Divergence between samples of chimpanzee and human DNA sequences is 5%, counting indels Counting both, human and chimp DNA was about 95% identical in that sample.5PubMed Central. Divergence between samples of chimpanzee and human DNA sequences is 5%, counting indels The full chimp genome comparison found about 5 million such insertion or deletion events.6PubMed. Initial sequence of the chimpanzee genome and comparison with the human genome
The effect can be much larger in a small region. In one immune-gene region, 98.6% identity fell to 86.7% once inserted or missing stretches were counted.7PubMed Central. Comparative sequencing of human and chimpanzee MHC class I regions unveils insertions/deletions as the major path to genomic divergence
Both the high and low numbers are correct, because they answer different questions. A letter-swap count asks how often aligned positions disagree. A count that includes inserted or missing stretches asks how much of the total sequence has a counterpart in both species. The first suits tracing mutations along the family tree, and the second suits asking how much DNA is shared at all.
You may see the lower figures used to argue that humans and chimps are less related than claimed. They measure a different thing, not a weaker relationship.
The Big Structural Differences Between Our Chromosomes
Chimps have 48 chromosomes and humans have 46. Human chromosome 2 formed when two ancestral chromosomes fused head to head, and they remain separate in other apes.8PubMed Central. Genomic Structure and Evolution of the Ancestral Chromosome Fusion Site in 2q13–2q14.1 and Paralogous Regions on Other Human Chromosomes One estimate dates the fusion to about 900,000 years ago, with a range of 400,000 to 1.5 million years.9PubMed Central. Revised time estimation of the ancestral human chromosome 2 fusion
The fusion left traces in our own DNA. Chromosome tips carry a repeated end-cap sequence, and the fusion point in chromosome 2 still carries that repeat, right in the middle of the chromosome where two ends once met.10PubMed. Origin of human chromosome 2: an ancestral telomere-telomere fusion Chromosome 2 also holds the remains of a second pinch point, the narrow spot every chromosome has, which is just the kind of leftover you would expect from two chromosomes joining into one. Together these let researchers confirm the fusion from human DNA alone.
The fusion changed the packaging more than the contents. The genes on chromosome 2 are largely the same genes found on two chimp chromosomes.
Flipped segments also separate us. An inversion is a stretch of a chromosome that was cut out, turned around, and put back, so the letters are the same but read in the opposite direction. Nine large flipped segments around chromosome centres distinguish human and chimp chromosomes, and seven of those nine happened in the chimp line, not ours.11PubMed. The chimpanzee-specific pericentric inversions that distinguish humans and chimpanzees have identical breakpoints in Pan troglodytes and Pan paniscus Genes inside an inverted segment can still work, though the change can affect how neighbouring regions behave. Inversions can also interfere with chromosomes pairing up when eggs and sperm are made in hybrids, so such flips may have helped the two lineages stop interbreeding.12PubMed. Molecular definition of pericentric inversion breakpoints occurring during the evolution of humans and chimpanzees
How Small DNA Changes Make a Big Difference in the Brain
Many of the changes that matter most sit in DNA switches rather than in genes themselves. Some of these are stretches that barely changed across mammals but changed quickly in humans after we split from chimps.13PubMed Central. Comparative characterization of human accelerated regions in neurons Most don’t code for proteins, so they don’t alter what genes make. They alter the instructions for using them. Many sit near genes that guide brain development.14PubMed Central. Comparative characterization of human accelerated regions in neurons The same gene used longer during brain growth could yield a larger brain.
At least 30% of such regions likely act as switches that turn genes on during development.15PubMed Central. Many human accelerated regions are developmental enhancers To test one, researchers place the human or chimp version of a switch next to a marker gene in a mouse embryo and see where and how strongly the marker turns on. Five of these switches behaved differently in their human and chimp versions.16PubMed Central. Many human accelerated regions are developmental enhancers That shows the switch works differently in a living, developing body. It doesn’t show that the difference makes a human brain, since mice aren’t a model of human thinking and only five switches were reported to differ. In brain stem cells, these switches control about 2,963 genes involved in building and wiring nerve cells.17PubMed Central. Resolving the three-dimensional interactome of Human Accelerated Regions during human and chimpanzee neurodevelopment
And because a switch is specific to a tissue, changing it can alter the brain without breaking the gene elsewhere in the body.
About 10% of genes differ in how strongly they are switched on in at least one brain region.18PubMed Central. Regional Patterns of Gene Expression in Human and Chimpanzee Brains A small set of interacting control genes may coordinate many of these brain differences.19PubMed Central. Differences in human and chimpanzee gene expression patterns define an evolving network of transcription factors in brain
Oddly, the brain shows the fewest differences of the tissues compared, while the liver shows the most.20PubMed. Parallel patterns of evolution in the genomes and transcriptomes of humans and chimpanzees Brain tissue is built from many specialised cell types that depend on tightly regulated genes, so large changes there could be harmful and tend to get filtered out. The liver deals more directly with diet and toxins, which differ between species, so more change in activity may be tolerated or even favoured.
Specific Genes That Set Humans Apart
FOXP2 is a gene involved in speech and language.21PubMed. Molecular evolution of FOXP2, a gene involved in speech and language It stayed nearly unchanged across mammals until the human line gained two altered protein building blocks, a tiny change in a protein of hundreds.22PubMed Central. Human-Specific Transcriptional Regulation of CNS Development Genes by FOXP2 In human nerve cells, the human version switched about 116 genes up or down differently from the chimp version.23PubMed Central. Human-Specific Transcriptional Regulation of CNS Development Genes by FOXP2
People with damage to FOXP2 have trouble with speech and language, which is how the gene was linked to them. But FOXP2 is not a single “language gene.” It is one piece among many, and it didn’t create human language by itself.
Other changes involve copying. Some genes gained extra copies only in humans, which may account for human-specific traits.24PubMed. The role of human-specific gene duplications during brain development and evolution When a gene is copied, the extra copy can drift and take on a new role while the original keeps doing its job. Human-specific copies of brain-growth genes are of interest because they may change how many cells are made or how long the brain develops. Changing the dose of one such gene, GPR89B, made zebrafish brains smaller or larger.25PubMed Central. Human-specific gene expansions contribute to brain evolution A fish brain isn’t a human brain, so this demonstrates a mechanism rather than proving the human effect.
Humans also lost genetic material. We lost about 510 stretches of DNA that other primates kept, and several genes stopped working in humans, including an immune gene.26PubMed Central. Differences between human and chimpanzee genomes and their implications in gene expression, protein functions and biochemical properties of the two species A broken gene isn’t always harmful. It can remove a function no longer needed, and some losses may simply have drifted in without consequence.
Are We Closer to Chimpanzees or Bonobos
Bonobos and chimps split from each other long after our line split from theirs. They share a more recent common ancestor with each other than either does with us, so both are equally closely related to humans. Calling one our closest relative to the exclusion of the other isn’t accurate.
Small parts of our genome still lean one way or the other. About 2.52% of the human genome is closer to bonobos and 2.55% closer to chimps.27UW Medicine | Newsroom. New bonobo genome fine tunes great ape evolution studies These are the shares found by checking, stretch by stretch, which ape matches us better. The two numbers are almost the same, which is what you would expect if neither ape is closer overall. In the remaining roughly 95%, the two apes are so close to each other that neither is favoured.
The family tree is patchy more widely, too. About 30% of DNA letters don’t follow the expected family-tree pattern between humans, chimps and gorillas.28PubMed Central. Insights into hominid evolution from the gorilla genome sequence Before species split, an ancestral population carries many versions of each gene, and which versions end up in which descendants happens partly by chance. When two lineages split in quick succession, this sorting is incomplete, so some stretches of a genome follow the species tree and others don’t.
What DNA Differences Mean for Health
Humans and chimps share over 99% of the protein sequences that can be compared.29PubMed Central. Relative Over-Reactivity of Human versus Chimpanzee Lymphocytes: Implications for the Human Diseases Associated with Immune Activation Even so, humans infected with HIV or hepatitis C tend to develop stronger reactions and long-term complications than chimps.30PubMed Central. Relative Over-Reactivity of Human versus Chimpanzee Lymphocytes: Implications for the Human Diseases Associated with Immune Activation Chimps infected with these viruses often show milder disease, which suggests their immune cells react less aggressively. Human immune cells carry fewer braking molecules, the surface molecules that tell a cell to calm down once a threat has been dealt with.31PubMed Central. Relative Over-Reactivity of Human versus Chimpanzee Lymphocytes: Implications for the Human Diseases Associated with Immune Activation With fewer brakes, human immune cells may stay activated more easily. This is a difference in the immune system’s settings rather than in which proteins exist.
A protein sequence can be over 99% shared and still behave differently if small changes affect where or how much of it is made. In lab tests, genes that responded to a bacterial trigger only in humans were rich in immune-disease genes.32PubMed Central. Functional Comparison of Innate Immune Signaling Pathways in Primates
This matters for medical research. Chimps were once used to study human viruses because of their genetic closeness, but differences in immune response mean results don’t always transfer.