A single genetic tweak caused human ancestors to lose their tails 25 million years ago — but it came at a cost

• Scientists have found a mutation in our DNA that snipped our tails off.
• The mutation may have allowed our ancestors to walk upright.
• But it may also be behind a birth defect that still affects babies today.

We may have finally figured out why we begin developing tails in the womb — and why we lose them at around eight weeks of gestation.

A single genetic tweak that occurred among our ancestors 25 million years ago means humans today are unable to grow a tail, according to a new study.

This tweak to our genetic makeup may have given us evolutionary advantages, such as being able to walk upright — but it came at a cost.