The Chromosome Count Discrepancy
Humans have 46 chromosomes (23 pairs), while chimpanzees, gorillas, and orangutans—our closest living relatives—have 48 chromosomes (24 pairs). This difference presents a challenge for the theory of common ancestry: how could humans have evolved from a common ancestor with these species if we have different chromosome counts?
Key Fact
All great apes (chimpanzees, gorillas, orangutans) have 48 chromosomes, but humans have only 46.
| Species | Chromosome Count | Closest Living Relative |
|---|---|---|
| Humans (Homo sapiens) | 46 (23 pairs) | — |
| Chimpanzees (Pan troglodytes) | 48 (24 pairs) | Humans (~98.8% DNA similarity) |
| Gorillas (Gorilla gorilla) | 48 (24 pairs) | Humans and chimps |
| Orangutans (Pongo pygmaeus) | 48 (24 pairs) | Humans, chimps, and gorillas |
In 1982, scientists hypothesized that if humans evolved from a common ancestor with the other great apes, two ape chromosomes must have fused to create human chromosome 2, reducing the chromosome count from 48 to 46.
Evidence of Chromosome Fusion
1. Chromosome Banding Patterns
When stained, human chromosome 2 shows banding patterns that match chromosomes 2p and 2q (2p and 2q refer to "chromosome 2-predecessor" and "chromosome 2-subsequent"), which correspond to two separate chromosomes in apes.
2. Telomere Sequences
Telomeres are distinctive DNA sequences found at the ends of chromosomes. Human chromosome 2 contains telomere sequences in the middle, exactly where the fusion would have occurred. These sequences consist of repeats of TTAGGG, which are characteristic of telomeres but should never be found in the middle of a chromosome unless a fusion has occurred.
3. Additional Centromere
Every chromosome has a centromere, a structure that plays a key role in cell division. Human chromosome 2 contains remnants of a second, inactivated centromere precisely where expected if two chromosomes fused.
Scientific Confirmation
This hypothesis was definitively confirmed through multiple lines of evidence:
- In 2002, researchers identified the exact fusion site on chromosome 2 at position 2q13. Direct sequencing of the fusion site revealed head-to-head fusion of telomeric DNA, with over 150 repeats of the telomere sequence TTAGGG.
- The 2005 completion of the chimpanzee genome confirmed that human chromosome 2 aligns perfectly with two chimp chromosomes (2A and 2B).
- In 2005, researchers identified the precise remnants of the second centromere from one of the ancestral chromosomes.
Why This Is Strong Evidence for Evolution
Human chromosome 2 provides compelling evidence for evolution for several reasons:
1. Predictive Power
Scientists predicted the fusion based on comparative chromosome counts, then found exactly what they expected. This demonstrates the predictive power of evolutionary theory.
2. Unnecessary Design
There is no functional reason for human chromosome 2 to contain vestigial telomere sequences in the middle or a second inactivated centromere. These features make sense only as remnants of our evolutionary history.
3. Shared Genetic History
The content and organization of genes on human chromosome 2 match those found on the two separate ape chromosomes, confirming our shared genetic history.
4. Clear Mechanism
Chromosome fusion is a well-documented genetic process that occurs in many species. This specific fusion explains precisely how humans evolved with 46 chromosomes from ancestors with 48 chromosomes.
Conclusion
Human chromosome 2 represents one of the clearest examples of a testable evolutionary prediction that has been thoroughly confirmed. The evidence shows beyond reasonable doubt that two ancestral ape chromosomes fused to form human chromosome 2, providing powerful molecular evidence for our shared ancestry with other great apes.
This chromosome fusion event occurred after the evolutionary split between humans and chimpanzees, estimated to have happened 4-6 million years ago, and has become fixed in the human population.