Why Don't We Have Tails?

 Some people wish we did have tails. Anyone who goes to SF cons has seen the person wandering around with ears and a tail.

But like all apes, we only have the coccyx or tailbone, a small set of vertebrae that extend just beyond the pelvis. Our tail is completely vestigial and invisible.

And now we've found the genetic underpinning. It's a mutation in a gene called TBXT...and unlike many mutations that cause taillessness (Manx cats, for example, have a lot of back pain), it doesn't seem to cause any issues. You can use genetic engineering of this gene to breed tailless mice that do just fine.

Except that doesn't answer the question.

What we don't quite know is what evolutionary advantage not having a tail gave to our ancestors. Gibbons, after all, have truly splendid appendages. The common wisdom is that we don't bother to grow one because it's not useful, but that doesn't quite explain it. Very few mammals are naturally tailless.

At some point in the transition from monkey to ape, tails got in the way...but we still don't quite understand why.