Chimpanzees have 48 chromosomes, arranged as 24 pairs, while humans have 46.1PubMed Central. Differences between human and chimpanzee genomes and their implications in gene expression, protein functions and biochemical properties of the two species That doesn’t mean chimps carry extra genetic material. The difference comes from how the same material is packaged. The change also happened on the human side, which makes our chromosome count, not the chimp’s, the unusual one among great apes.2John Hawks Weblog. When did human chromosome 2 fuse?
What a Chimpanzee’s 48 Chromosomes Look Like Next to Ours
Every cell with a nucleus in a chimp’s body carries the same full chromosome set, with half from each parent. Laid side by side, the chimp and human sets look very similar overall.3PubMed Central. Differences between human and chimpanzee genomes and their implications in gene expression, protein functions and biochemical properties of the two species Most chimp chromosomes have a clear human counterpart with the same genes in largely the same order. Scientists first spotted these matches over 40 years ago by staining chromosomes so their banding patterns showed up under a microscope.4johnhawks.net. When did human chromosome 2 fuse? The sex chromosomes are part of the count too: both species use X and Y, with females carrying two X chromosomes and males an X and a Y.
Matching chromosomes are not identical. They carry the same genes in largely the same order, but the DNA letters differ in many places, so “match” means the same region of the genome, not the same sequence. That is why scientists can name a chimp chromosome by its human counterpart and use one species to study conditions in the other.
The major difference involves human chromosome 2.5PubMed Central. Differences between human and chimpanzee genomes and their implications in gene expression, protein functions and biochemical properties of the two species Its contents match two separate chimp chromosomes, labeled 2a and 2b.6PubMed Central. Differences between human and chimpanzee genomes and their implications in gene expression, protein functions and biochemical properties of the two species Two chimp chromosomes correspond to one human chromosome, and that accounts for the full gap of one pair.
The numbering adds to the confusion. Chromosomes are numbered roughly by size, so matching chromosomes can carry different numbers in each species. Chimp numbers were adjusted to 2a and 2b so they line up with the human system.
Chromosome number also says little about how much DNA an animal has or how complex it is. Some plants and fish have far more chromosomes than any ape.
Why Humans Have Two Fewer Chromosomes
Human chromosome 2 formed when two ancestral chromosomes joined head to head, while they stayed separate in other primates.7PubMed Central. Genomic Structure and Evolution of the Ancestral Chromosome Fusion Site in 2q13–2q14.1 and Paralogous Regions on Other Human Chromosomes They joined at their telomeres, the protective tips that cap the ends of every chromosome.8PubMed Central. Revised time estimation of the ancestral human chromosome 2 fusion So material that once sat at chromosome tips now sits in the middle of human chromosome 2.9PubMed Central. Genomic Structure and Evolution of the Ancestral Chromosome Fusion Site in 2q13–2q14.1 and Paralogous Regions on Other Human Chromosomes
A telomere is a repeated stretch of DNA at each chromosome end. It works like the plastic tip on a shoelace, keeping the end from fraying or sticking to other chromosomes. Cells normally treat a bare chromosome end as damage and try to repair it, which is why ends are capped. A head-to-head fusion therefore joined two capped ends and left the caps buried inside the new chromosome. The new chromosome still had its own outer ends, so it was protected like any other.
Scientists found a stretch of human chromosome 2 that still carries the remains of two chromosome tips fused together.10PubMed. Origin of human chromosome 2: an ancestral telomere-telomere fusion That spot marks the exact point where the two ape chromosomes merged.11PubMed. Origin of human chromosome 2: an ancestral telomere-telomere fusion Labs later confirmed this with glowing DNA probes that light up matching chromosome regions.12PubMed Central. Revised time estimation of the ancestral human chromosome 2 fusion
At the fusion site, the repeated tip DNA appears facing each other, as if two tips were pressed together. Normal chromosomes never carry tip repeats in their middle, so the pattern is a fingerprint of joining.
The fusion didn’t harm our ancestors, because essentially no genes were lost and the full set of instructions remained. One of the two original centers of chromosome 2 was silenced, leaving a working chromosome. A carrier of such a change can still have fertile children, which lets it spread over generations. Because all living humans share the chromosome 2 fusion and other apes do not, it happened after our line split from the chimp line and before modern humans spread.
Other Great Apes Have 48 Too
Chimpanzees, bonobos, gorillas, and orangutans all have 24 pairs of chromosomes.13John Hawks Weblog. When did human chromosome 2 fuse? So the 48-chromosome set is the ancestral pattern, and humans changed from it.14Center for Academic Research and Training in Anthropogeny (CARTA). Chromosome Number Comparing these ape genomes shows the fusion happened sometime during human evolution.15John Hawks Weblog. When did human chromosome 2 fuse? The separate pieces in other apes are smaller chromosomes with their center near one end, while human chromosome 2 is large with its center near the middle.16Center for Academic Research and Training in Anthropogeny (CARTA). Chromosome Number
The ape chromosomes that match human chromosome 2 have one long arm and one very short arm. Human chromosome 2 has two arms of more similar length, which is the shape you’d expect from joining two such chromosomes at their short ends.
A single change in the human line explains the pattern better than separate gains in four ape lines. Humans lost a pair by merging; apes did not add one.
Bonobos are chimps’ closest relatives and share their 48 chromosomes. Several chimp-specific chromosome rearrangements look identical in bonobos, showing they arose before the two split.
Other Ways Chimp and Human Chromosomes Differ
Besides the one fusion, the two chromosome sets differ by nine inversions, stretches that are flipped end to end.17PubMed Central. Molecular characterization of the pericentric inversion that causes differences between chimpanzee chromosome 19 and human chromosome 17 Seven of those nine flips happened in the chimp line, not ours.18PubMed. The chimpanzee-specific pericentric inversions that distinguish humans and chimpanzees have identical breakpoints in Pan troglodytes and Pan paniscus The same flips appear at identical points in chimps and bonobos.19PubMed. The chimpanzee-specific pericentric inversions that distinguish humans and chimpanzees have identical breakpoints in Pan troglodytes and Pan paniscus
A flipped segment is cut out, turned around, and put back, so the genes in it run in the opposite order. The genes stay the same, though genes near the break points can be switched on or off differently afterward.
A flipped segment is hard to line up with its normal partner, so the chromosomes can pair awkwardly when egg and sperm cells form. That is one reason rearrangements can help separate populations over time. Carriers may still be perfectly healthy if no gene is broken at the cut points, which is how such changes persist.
Many chimp and gorilla chromosomes also carry large extra caps of tightly packed DNA next to their tips.20PubMed Central. Epigenetic and evolutionary features of ape subterminal heterochromatin These caps are absent in humans and orangutans.21Genome Research. The evolution of African great ape subtelomeric heterochromatin and the fusion of human chromosome 2 They sit beside the telomere rather than being part of the tip, and because they take up space, many chimp chromosomes look slightly longer at the ends than their human matches under a microscope. Humans and orangutans have plainer ends.
The male Y chromosome is where chimps and humans differ most.22PubMed Central. Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content Only 14 to 27% of the human Y lines up with other ape Y chromosomes, compared with 93 to 98% of the X.23PubMed Central. The complete sequence and comparative analysis of ape sex chromosomes In one part of the Y, humans kept all 16 genes while chimps lost 4.24PubMed. Evolution of X-degenerate Y chromosome genes in greater apes: conservation of gene content in human and gorilla, but not chimpanzee One explanation is intense competition between sperm in chimps, since females mate with several males.25PubMed. Evolution of X-degenerate Y chromosome genes in greater apes: conservation of gene content in human and gorilla, but not chimpanzee
Can Chimpanzees Get Down Syndrome
Chimp chromosome 22 matches human chromosome 21.26PubMed. Chimpanzee Down syndrome: a case study of trisomy 22 in a captive chimpanzee The numbers differ because chromosomes are numbered by size, even though the contents line up. So a chimp with three copies of chromosome 22 has a condition comparable to human Down syndrome.27PubMed. Chimpanzee Down syndrome: a case study of trisomy 22 in a captive chimpanzee It carries an extra copy of nearly the same genetic material that causes the condition in people.
One such captive chimp showed slow growth, cataracts, eye problems, a heart defect, and missing teeth, all common in human Down syndrome.28PubMed. Chimpanzee Down syndrome: a case study of trisomy 22 in a captive chimpanzee An earlier reported infant chimp had slowed development, weak muscle tone, and loose joints.29PubMed. Autosomal trisomy in a chimpanzee: resemblance to Down’s syndrome
These cases are rarely seen. Wild chimps with serious birth conditions likely don’t survive long enough to be noticed, and known cases come from captive animals under veterinary care. Only a handful have been described, so how often it happens is unknown.
Can Humans and Chimps Interbreed With Different Chromosome Numbers
No human-chimp hybrid has ever been confirmed, despite rumors. Chromosome number alone isn’t an absolute block: horses and donkeys differ in chromosome number yet produce mules. Mules are usually sterile, though, which shows how mismatches cause trouble in the next generation.
If one parent carries a different arrangement, such as a fused chromosome that matches two partner chromosomes, chromosomes pair awkwardly when egg or sperm cells form. That can produce cells with missing or extra chromosome material, so fewer viable eggs or sperm result, which can reduce fertility but not always eliminate it.
Genes in rearranged chromosomes changed faster between humans and chimps.30PubMed. Chromosomal speciation and molecular divergence–accelerated evolution in rearranged chromosomes These rearrangements may have helped push the two lineages apart.31PubMed. Chromosomal speciation and molecular divergence–accelerated evolution in rearranged chromosomes But the human and chimp lines split roughly 6 million years ago, and many other differences built up since.32PubMed Central. Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content