NARRATOR: This three-week-old pig embryo already shows signs of a growing tail. In two more weeks, the tail has developed further. In its early stages, a chick embryo also shows promise of a tail, but just a few days later, the tail is gone. At four weeks, a human embryo also shows indications seen at the right.
Traces of a tail occur in each embryo, mementos of a heritage we all share.
At six weeks, though, the tail is just a small vestige at the spine's end. And one week later, it has dwindled to a bud, seen just beneath the legs.
Animals that lack tails share a common ancestry with those that have them. Our shared ancient beginnings on land and in water link us together, despite our ultimate differences.
Human limbs begin as buds, seen as early as five weeks in this embryo. At this stage, an arm bud, seen here, appears very similar to this leg bud. By ten days, the limb buds are shaped like tiny paddles,and distinctions between arm and leg buds are visible.
The distinctions can seem slight. A human arm, at this stage, still looks like a pig's embryonic front leg. Bird embryos at this stage show extraordinary similarities to those of humans. The left bud, looking so much like a human arm, will ultimately develop into a wing.
Each species starts with a shared beginning... but takes a distinct course, acquiring distinguishing traits of its own.
The embryo of a chicken follows its own path. Human-like at first, the pig embryo soon takes on its own distinctive form, with snout and tail. The fish embryo soon differs profoundly from a human...reflecting an evolutionary course that diverged on its own long, long ago. Sharing ancient beginnings with these others...the human embryo.
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