Sexual dimorphism
Humans show a degree of sexual dimorphism - males, on average, are larger than females. This is true for most mammalian species, but the situation is often reversed in other orders - in spiders, for example, the female is usually the larger sex.
The origins of sexual dimorphism are not completely understood, but there seems to be a link between low levels - as seen in humans - and greater input into child rearing by the male.
Sexual selection may also play a role. In many species, males compete with males for the attention of females, who choose their mates. This can lead to the evolution of parts of the body that are of no selective advantage but signal fitness to females. Ornamentation is used by males to display their quality (the peacock), while weaponry enables males to establish authority in combat (in stags, for example).
Some forms of sexual dimorphism are extreme. In anglerfish, the tiny male attaches to the side of a female semi-parasitically, doing little more than provide the female with sperm. In green spoon worms, the female actually engulfs the male who resides in the female's digestive tract, churning out sperm.
In early hominids, though, sexual dimorphism may have been more pronounced. Studies of Paranthropus robustus (pictured above), which appeared around two million years ago, suggest that males were nearly 20 per cent bigger than females. This may have reflected polygamy - harem building, as seen in today's silverback gorillas.
The extra size may have helped adult males beat off challenges from other males. The downside of this emphasis on bodybuilding may have been a vulnerability to predation - Paranthropus bones have been found in caves used by large carnivores. The low levels of human sexual dimorphism may therefore be a fairly recent evolutionary innovation, arising within the past couple of million years.
Image: Sergio Perez/Flickr