Evolutionary Proof. Content Warning.
Your body is a temple, but it also has a museum of natural history. Look closely and you'll see parts that aren't there because you need them but because your animal ancestors did. No longer serving their previous function but not costly enough to have disappeared, these remnants of our deep history only make sense within the framework of evolution by natural selection. With your arm on a flat surface, push your thumb against your pinky and tip your hand slightly up. If you see a raised band in the middle of the wrist, you've got a vestigial muscle in your forearm. That tendon you see connects to the Palmaris longus, a muscle that around 10-15% of people are missing on one or both of their arms. It doesn't make them any weaker though. There's no difference in grip strength. In fact, it's one of the first tendons that surgeons will take out so they can use it in reconstructive and cosmetic surgeries. You can find the Palmaris longus across mammal species, but it's most developed among those that use their forelimbs to move around. In primates, that means the muscle is longer in lemurs and monkeys and shorter in chimps, gorillas, and other apes that don't do a lot of scrambling through trees. It's not the only leftover muscle that we've got. Look at the three that are attached to our outer ear. We can't get much movement out of these muscles, especially compared to some of our mammal relatives who use them to locate the sources of sounds. Presumably this would have been quite helpful for early nocturnal mammals.
