Endogenous Retroviruses

ERVs as Evidence for Evolution

Endogenous retroviruses provide several lines of evidence supporting evolutionary theory:

Common Ancestry: Shared ERVs between species indicate a common evolutionary origin. For example, humans and chimpanzees share over 200 individual ERV insertions into the genome, suggesting a recent common ancestor. See this video by Stated Clearly for a great presentation on this topic.
Adaptive Evolution: Some ERVs have been co-opted for host functions, illustrating how evolution can repurpose genetic material.
Molecular Clock: The degree of sequence divergence in ERVs can be used as a "molecular clock" to estimate the time of species divergence.

Examples of ERVs

Human endogenous retrovirus K is found in humans and other primates. Its distribution and sequence variations across primate species align with the established evolutionary relationships.
Syncytin Genes: These genes, derived from ERVs, are crucial for placental development in mammals. Different mammalian lineages have independently co-opted different ERVs for this function, demonstrating convergent evolution.
Koala Retrovirus (KoRV): This virus is currently in the process of becoming endogenous in koalas, providing a real-time example of how ERVs become integrated into host genomes.

Improbability of Shared ERVs

The shared presence of endogenous retrovirus (ERV) insertions in humans and chimpanzees at identical genomic locations poses a significant challenge to non-evolutionary models. Let's examine this mathematically.

Key Facts:

Human genome size: ~3 billion base pairs
Shared ERV insertions between humans and chimps: >200
Not all genomic locations are suitable for insertion. Let's conservatively estimate that 1% of the genome is insertable.

Calculation:

Probability of a single ERV inserting at a specific location:
$P(\text{insertion}) = \frac{1}{3 \times 10^9 \times 0.01} = \frac{1}{3 \times 10^7} \approx \frac{1}{30,000,000}$
Probability of the same ERV inserting at the same location in both species independently:
$P(\text{shared insertion}) = P(\text{insertion})^2 = \left(\frac{1}{30,000,000}\right)^2 \approx \frac{1}{900,000,000,000,000}$
Probability of 200 shared insertions occurring independently:
$P(\text{200 shared insertions}) = \left(\frac{1}{900,000,000,000,000}\right)^{200} \approx \frac{1}{10^{3000}}$

Interpretation:

The probability of 200 shared ERV insertions occurring independently in humans and chimpanzees is astronomically small (

\frac{1}{10^{3000}}

). For comparison:

Number of atoms in the observable universe: $~10^{80}$
Probability of randomly selecting the same atom from the universe 37 times in a row: $~\frac{1}{10^{2960}}$

Thus, the probability of shared ERV insertions occurring by chance is even lower than this already unfathomable scenario.

Explanation:

The evolutionary model explains these shared insertions simply:

ERVs inserted into the genome of a common ancestor.
These insertions were inherited by both human and chimpanzee lineages.

This explanation requires no statistically improbable events and aligns with other genetic and fossil evidence of common ancestry.

Conclusion:

The mathematical improbability of shared ERV insertions occurring independently provides strong support for the evolutionary model of common ancestry between humans and chimpanzees.