Let's see if we can get to the root of some of you hair questions (sorry about that). I'm a mammalogist (someone who studies mammals) so hair questions are interesting to me. All mammals have hair at some point in their lives (whales have hair before they're born). What other characteristic do all mammals have? Fur and hair are the same thing, we just use different words for it. There are different types of hair. The hair that makes up your eyelashes is different from the hair on your head or the very fine hair on the back of your hand. If you look at a cat or dog, you'll see that they have different types of hair too. They usually have a thick, wooly under-coat covered by longer, thicker hairs. They have "whiskers" on their face that help them sense movement in the dark.

So why is your dog's coat a certain length? Take a look at the hair on your arm. You will notice that it only gets to a certain length and stops. All hair grows for a certain amount of time, then stops. The hair on a human head just grows for a longer time than dog hair or human arm hair. Human scalp hair grows about 0.35 mm per day. The hair follicle stays active, producing the hair, for about 500-800 days. How long can the hair on your head grow? How fast any one person's hair grows, and how long it keeps growing is determined by that person's genes.

Why did humans lose most of their body hair while apes did not? We actually still have hair everywhere except for the palms of our hands, the soles of our feet, and our lips. You and I have about as many hair follicles as a chimp has, but each of our hairs (except for the ones on our heads) is a lot shorter. Why is our hair so much shorter? I usually think about these questions by thinking about what's good and bad about something (costs and benefits). We think humans evolved in hot, sunny places. Hair would help to protect us from the sun, but would be more likely to give parasites (ticks, fleas, and lice, for example) a place to hide. Losing most of our hair would reduce the parasites we carried around. We could have protected ourselves from the sun in other ways, such as wearing clothing. Head hair would keep the sun off of us where we'd get the most sun. No one really knows, though. What do you think?

Why did hairy animals evolve? Mammals and birds are different from most animals because we create body heat rather than depending on heat from outside our bodies. The word for this is "endothermic," but a lot of people call it being "warm blooded". Making all this heat takes a LOT of energy, so we need insulation to keep it in. Fat is a good insulator. So are hair and feathers. Why do you think marine mammals like whales and dolphins use fat instead of hair for insulation?

I assume that you are referring to the hair on your head. While it is indeed true that the hair on your head grows the longest of any of that on the remainder of your body, virtually all body hair is genetically predetermined to grow a certain length. You can easily test this by shaving off your eyebrows and watching them re-grow to their current length (I do not, however, recommend this as eyebrow-less people look a little strange. Just look at Mona Lisa. Perhaps she too was interested in this question yet failed to realize the time required for her eyebrow hair to re-grow before scheduling her portrait painting session). You could also shave off a small patch of arm hair and observe a similar effect. Also, if you were to shave your dog, its hair would grow back to approximately its original length.

Why did humans loose most of their body hair while apes did not? While there is not a clear-cut answer to this question, there was obviously some evolutionary pressure that selected against extensive hairiness in early humans. Since it is suggested that early man may have evolved in tropical Africa, one can imagine that excessive hairiness might have been a major disadvantage as man began to colonize dryer and hotter climates. With a gradual evolutionary loss of large quantities of body hair, there was also a subsequent increase in solar exposure and an increased susceptibility to skin cancer. To counteract this, natural selection favored survival of humans with increased skin melanin concentration, which acted as a natural sunscreen, protecting these early people. As humans began to colonize the rest of the world and subsequently invade colder climates, there was less of a demand for large concentrations of dermal melanin and subsequently, over evolutionary time, these people reverted to a lighter skin color. This is, however, only one possible explanation and several alternative hypotheses have been put forth to explain this. Most apes probably lacked the intellectual capacity to survive in unfamiliar habitats and thus, there was only a demand to retain the necessary quantity of hair to survive in the climate to which they were pre-adapted.

Why did evolution make hairy creatures? Hair serves several important functions, but one of its most important is that of an insulator. Furry creatures are better adapted for invading colder climates or nocturnal habitats (remember that furriness often goes along with warm-bloodedness). Those species with the most hair (or that functioned as the best insulator) were able to survive colder temperatures and eventually out-compete their hair-challenged counterparts, assuring survival of the species and the appearance of hairy individuals in future generations.

Wow, you asking the tough "why" questions here! Let's start with number 1: is there really a difference between fur and hair? At a basic level, "no." All mammals have hair (well, except for adult cetaceans -- whales and dolphins -- who actually lose their baby hair and replace it with layers of insulating blubber ). The "hair" has varying degrees of thickness, color, texture, and structure. And depending on these characteristics, we call it "fur" v "hair." Your dog's hair does grow -- but it falls out and is replaced before it gets too long. Did you see the movie "Lord of the Rings" or read the book or the prequel, "The Hobbit"? (By the way, I liked the movie, but the books are even better!) If so, you read/saw that hobbits have "furry" feet. Their feet have "hair" on them but because of how that hair looks, the word "furry" is used. It just gives you a different idea of what the hair is like.

Now we get to these "why" questions. It is really, really difficult to be certain about "why" something might have come about. But let's think it through --the "simplest" explanation is what we want to start with as a scientist. Neither dogs nor humans have always lived in warm, protected houses, right? Dogs rely on their fur coats for warmth and protection from the elements and dogs have been bred to "select for" certain types of coats (compare and English sheep dog with a chocolate lab). Apes rely on their fur for protection and warmth, too. Most loss of body heat in a human is through the head (where our big brains are being supplied lots of warm blood close to the surface). We still have hair on our heads... Do you see where this is going? We cannot ever "prove" that humans lost most of their body hair because they no longer needed it to stay warm and protected, but we can hypothesize about it.

There are other possibilities as well -- think them all through and then stick with the simplest one. If you can disprove that hypothesis, then move to the next one. It's the way science works! You always disprove things as opposed to having absolute proof of something. And keep asking those tough questions!