The Tale of Dead Genes: Tracing Evolution through Our Genetic Graveyard

Our DNA serves as a historical record, tracing not only our species but also our ancestors over millions of years. Among the active genes that define us are numerous 'dead' genes or pseudogenes, once functional but now inactivated by mutations. These remnants provide insights into our evolutionary past.

The Story of Pseudogenes

Pseudogenes are like fossils within our genome. They arise when a gene undergoes mutations that disrupt its function, often through insertions, deletions, or point mutations. These defunct segments of DNA are no longer under the same selective pressure as functional genes, so they accumulate more mutations over time (1).

A mutation in the GLO (also known as GULO) gene disrupt synthesis of a protein necessary to make vitamin C. See description below for more details. * = mutation

Examples of Pseudogenes

• The GLO/GULO Gene: Vitamin C Synthesis and Dietary Changes Most mammals have the capability to synthesize vitamin C, thanks to the functioning of the GLO (gulonolactone oxidase) gene (also known as GULO). In humans, however, this gene is a pseudogene, rendering us unable to produce vitamin C (image above). This change necessitates the intake of vitamin C through our diet. Nishikimi et al. suggest this mutation arose in our primate ancestors during a period abundant in vitamin-C-rich fruits. Interestingly, this mutation in the GLO gene seems to overlap with the development of trichromatic vision in primates, suggesting a fascinating evolutionary interplay between diet, environment, and genetic changes (2).
  

• Olfactory Receptor Genes and the Evolution of Human Senses Humans possess approximately 1,000 genes for olfactory receptors, but more than half of these are non-functional pseudogenes, This prevalence of pseudogenes among olfactory receptors suggests that over time, humans have become less reliant on the sense of smell. A study by Gilad et al. highlights this reduction as potentially linked to the development of enhanced vision in our ancestors. In contrast, many of these genes remain active in animals like mice, providing them with a more acute sense of smell. This sensory trade-off is an intriguing aspect of human evolution, where the development of one sense may have led to the diminishment of another (3).

• Evolutionary Remnants: From Whales to Humans Evolution leaves traces in our genomes in the form of pseudogenes. For instance, whales have pseudogenes for hair proteins, reflecting their evolutionary transition from land-dwelling to aquatic mammals (4). Similarly, mammals, including humans, carry remnants of genes related to egg yolk production, such as the vitellogenin genes, which are a vestige of our egg-laying ancestors (5). Additionally, genes involved in tail formation, such as the T-box gene family, are present as pseudogenes in apes and humans, signifying an evolutionary loss of external tails in these species (6). See this blog post about tail loss in humans and apes.

These genetic remnants are like echoes from our evolutionary past, providing insights into the complex journey of life on Earth.

Research and Perspectives by Sean B. Carroll

Sean B. Carroll, a prominent evolutionary biologist, has shed light on the significance of these genetic remnants. In his books "The Making of the Fittest" (7) and "Endless Forms Most Beautiful," (8) Carroll explores how the study of genes, including pseudogenes, helps us understand evolutionary processes. He explains how these genetic 'fossils' provide irrefutable evidence of common ancestry and evolution.

The Role of Pseudogenes in Evolution

Pseudogenes are not just evolutionary leftovers. They serve as a genetic archive, helping scientists understand how species have adapted (or not) to changing environments. While pseudogenes may not produce functional proteins, their presence and state can inform us about historical selective pressures and environmental changes. It is worth noticing that some pseudogenes are transcribed into RNA, but are not translated into proteins suggesting a regulatory role of these RNA molecules (9).

Summary

Pseudogenes offer a unique perspective on evolution, acting as a molecular footprint of our past. As we delve deeper into our genetic makeup, these silent genes continue to tell stories of our ancestors, the paths they traveled, and the choices nature made in the grand saga of life.

References and further reading

1. Pseudogenes: Are they "junk" or functional DNA? Annual Review of Genetics.

2. Molecular basis for the deficiency in humans of gulonolactone oxidase, a key enzyme for ascorbic acid biosynthesis. The American Journal of Clinical Nutrition

3. Loss of olfactory receptor genes coincides with the acquisition of full trichromatic vision in primates. PLOS Biology.

4. Characterization of hairless (Hr) and FGF5 genes provides insights into the molecular basis of hair loss in cetaceans. BMC Evolutionary Biology

5. Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and Placentation. PLOS Biology

6. On the genetic basis of tail-loss evolution in humans and apes. Nature

7. Carroll, S.B. Endless Forms Most Beautiful: The New Science of Evo Devo.

8. Carroll, S.B. The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution.

9. Pseudogenes: Pseudo-functional or key regulators in health and disease? RNA