Why have humans evolved this trait? This New Scientist article outlines some of theories offered over the years.

We still don’t have a conclusive answer.

Theory 1: Avoiding the heat

As humans became savanna hunters body hair was shed to avoid overheating during long runs.

I have blogged before about theories suggesting that the human combination of hairlessness and profuse sweating allows for persistence hunting.

The problem:

Why has no other mammal adopted this strategy?

Many other mammals would benefit from better cooling systems and they all have fur – what is so different about us? Fur actually insulates against heat and the sun as well as keeping us warm in the cold.

Theory 2: Side-effect of big brains

This theory suggests that nakedness is a side effect of growing larger brains. It was noted that:

• Human brains grow very rapidly just before birth
• Humans retain characteristics of juvenile apes (flatter faces, bigger heads and eyes and naked skin).

The idea is that the stage just before birth has been protracted to allow the brain to grow even larger.

The problem:

If having hair were an advantage then we would just grow hair later in life. For example, we grow teeth even though we aren’t born with teeth.

Theory 3: No ticks here

Another theory is that we lost our hair in order to avoid ticks and other parasites.

The problem:

Why only us? If this were really an advantage other mammals would have evolved the same thing.

Theory 4: Aquatic ape

Humans are hairless and have a thin layer of fat under their skin. Sounds like whales and dolphins right?

The idea is that our evolutionary ancestors went through a stage evolving in aquatic environments. This could also help to describe why we walk upright.

The problems:

There are many problems with this theory (read about them on Wikipedia). For example, humans can’t hold their breath very well and are not efficient swimmers.

Most importantly, hairlessness is only an advantage in fully aquatic species like dolphins and whales. Even full time swimmers like otters and seals have thick fur and swim superbly. In short, the aquatic ape theory doesn’t work all that well…

It seems that was the case with Willy (aka Keiko) the star of the movie Free Willy. New Scientist outlines the story in this article.

Summary:

• The whale’s name was Keiko
• He was captured in Icelandic waters in 1979 at about 2 years old
• He then spent 10 years alone in a tank in Mexico
• After the 1993 public pressure mounted to free Keiko and in 2000 he was transferred back to Iceland
• He lived in a pen and was trained to swim out to sea with his trainers
• He briefly interacted with wild orcas but stayed away from them in general
• After his initial release he turned up in his pen 10 days later with an empty stomach (they tested)
• He eventually migrated to Norway but started seeking out humans and soon became overwhelmed by the attention he generated
• He was taken back to his pen in Iceland where he lived out his days. Even though the pen was open to the ocean he never left the bay again until his death in 2003

What are pheromones?
Actually, there is quite a lot of debate about that. Basically, pheromones are chemical messages emitted by animals that trigger responses in receivers.

For example a female moth will release a sex pheromone to signal that she is fertile. Male moths can detect and will react to the pheromone message.

There are many types of pheromones (see the Wikipedia article) including alarm pheromones, sex pheromones and food trail pheromones. They are actually fairly common in the animal world.

Pheromones in humans
Humans are animals so it would make sense that we would also produce and react to pheromones. They are evolutionarily useful after all.

For a variety of reasons (discussed in the article) there has been controversy about the existence of human pheromones. To me it seems pretty obvious that something is going on in humans too, and that is what the article is about.

Human examples
There are several examples of the unconscious effects of human pheromones:

• It has been shown that women living together will gradually synchronize their menstrual cycles. Later experiments showed that the sweat of women in different stages of their menstrual cycle could affect the cycles of other women. Sounds like pheromones.
• Brain scans have shown the ‘sex centre’ of women’s brains lighting up when they were exposed to the smell of a male sex hormone (found in men’s sweat). Interestingly the effect was only reliable when there was a man in the room.
• Alarm pheromones can also be subconsciously detected by humans. Test subjects were able to distinguish between the sweat of people who had watched scary movies and people who had watched funny movies. This was despite not being able to consciously tell the difference.
• In a similar experiment brain scans showed the fear centre of the brain lighting up when subjects were exposed to the sweat of first time skydivers.

Very interesting stuff. Luckily the effects seem pretty weak because it could get crazy if we learned to reliably affect people’s behavior using chemicals!

I’ve previously blogged about another experiment showing that fertile lap dancers earn more tips – this is just another example of the fact that we can subconsciously detect fertile women.

I explain the evolutionary reasoning behind these interesting effects in that article. In short, it pays women to conceal when they are fertile – so men will stick around all the time to be sure. It pays men to know when women are fertile so that they can focus energy when it counts.

Anyway, the article is actually about how a species of birds in the UK known as great tits have responded to global warming by laying their eggs earlier. They need to time their laying pretty precisely and over the last 30 years their laying has shifted 2 weeks earlier in the spring.

So here is that picture of some young great tits: