Hidden in your genome is a gene for vitellogenin — the protein that forms egg yolk. It doesn't work. It's been broken for about 30 million years, ever since our mammalian ancestors switched from laying eggs to using a placenta. But the fossil of the gene remains.

This is what evolutionary biologists call a pseudogene — a genetic relic that no longer functions but hasn't been deleted. Your genome is littered with them. You have broken genes for building an egg tooth (the small bump reptile embryos use to crack out of shells), for tasting certain compounds in decaying meat, for making your own vitamin C. Guinea pigs have the same broken vitamin C gene, which is why they also need to eat fruit.

Pseudogenes are one of evolution's strongest fingerprints. If every species were designed independently, there'd be no reason for a human to carry a broken egg-yolk gene. But if we share a common ancestor with chickens — an ancestor who laid eggs about 300 million years ago — the broken gene is exactly what you'd expect.

Charles Darwin didn't know about DNA, but he predicted something like pseudogenes in 1859. He called them 'vestigial organs' — structures that had lost their function but hadn't disappeared. The human appendix, the vestigial wings of flightless birds, the tiny pelvis bones in whales. Each one tells a story of evolutionary history written in bodies.

DNA takes this further. Your genome contains roughly 20,000 working genes and about the same number of broken ones. Viruses have even integrated themselves into our chromosomes and stayed there. About 8% of your DNA comes from retroviruses that infected our ancestors tens of millions of years ago. Most are dormant, but some have been repurposed. The gene that allows placental mammals to form a placenta? It came from a virus.

We often think of evolution as purposeful or progressive. It isn't. It's tinkering — adding, breaking, recycling, and occasionally stumbling onto something new. Your body isn't a designed machine. It's a 3.8-billion-year-old patchwork, every imperfection a clue to where you came from.