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Charles Darwin proposed his theory of evolution by natural selection in the early 19th century. At the time, scientists were just beginning to understand how species evolve and adapt to changes in their environment. Even today, our understanding of human evolution continues to change; perhaps the latest proof that we aren’t quite done evolving is the increasing prevalence of an extra artery in the arm.

The mysterious additional artery in question is the median artery, a lifeline for blood supply to the human forearm and hand during early human development. Traditionally, the artery has been considered an embryonic structure that forms in the mother’s womb and disappears around the eight-week mark. Under these circumstances, two arteries known as the radial and ulnar arteries then replace the median artery and take on the task of supplying blood to the forearm, hands and fingers.

However, recent discoveries have shown that some of us retain all three arteries well into our adult lives. What does the sudden presence of this additional artery mean for human development and the future of human evolution?