Last time we looked at how certain major adaptations such as hair loss have enabled humans to survive over the millennia in different conditions, and when faced with competition from other species. Not everything about the human body has a specific purpose, though, in the sense that we expect it to. One example of such thing is the philtrum - that little channel leading from the base of your nose to the upper lip. Recent research suggests that this development dates back millions of years, and has been inherited from fish. Apparently, when human embryos develop their face in the womb, all parts of the forehead, mouth, etc come together and fuse where the philtrum is located.
Some adaptations, on the other hand, are no longer relevant not because of their nature, but because the environmental selection pressure for which they evolved has disappeared. For example, an East Asian's typical eyelid shape evolved as a result of higher light intensities in that area of the world, yet the people born with this adaptation in other areas of the world simply don't need it. Another example is skin colour. Most physical adaptations due to local environments where separate human populations used to live for a very long time are now being mixed together because people can travel more easily around the world and settle.
There are of course, those properties which happen to be a certain way by chemical or otherwise coincidence, not evolution. For example, the colour of blood happens to be red due to the haemoglobin in red blood cells, which carry oxygen around the body. Similarly, there is no reason why we must have 10 fingers and 10 toes. We have merely used what our ancestors had, to adapt to new conditions. We have four limbs, but we walk upright. We still have 10 toes, but they've changed their shape (big toes).
Essentially, evolution is limited by whatever resources there are available. No organism can just grow wheels or another brain overnight, and that is why all organisms on Earth are so interlinked and similar. For example, the structure of the heart and brain can clearly be explained from birds to cats to humans, without any huge jumps in between, just small changes over a long period of time.
Check back next time for a different topic; we'll look at how positive and negative sexual selection work when it comes to relationships.