Humans have body plans that are bilaterally symmetrical. This means that if you draw a line down the centre of the human body, from the middle of the head to the feet, the same features appear on either side. We all have 2 eyes, one on either side of the body, and we also have 2 ears, 2 hands, 2 legs, 2 feet.
Bilateralism is such an important characteristic that it is used by scientists to classify living things. The group ‘Bilateria’ includes all bilateral organisms.
Bilateralism is a characteristic of vertebrates (mammals, reptiles, amphibians, fish, and birds) and also many invertebrates including the acoel, a type of flatworm.
The fossil record shows that species with bilateral symmetry lived as far back as 550 million years ago. The acoel was one of the first creatures to develop bilateralism.
Scientists are still working to understand more about the advantages of bilateralism. There are several possibilities.
A bilateral body plan makes movement easier, which helps animals to avoid predators and hunt for food. It may also favour a central nervous system, such as our spinal cord with its network of nerves leading up to the brain. Some studies suggest that animals look for symmetry when they are choosing a mate.
Whatever the advantages, bilateralism is a feature that unites humans with many other species, from acoel worms to birds and mammals. It is evidence that we are all related and that we share a common ancestor.
Not all living things are bilateral. Some don’t have any kind of symmetry in their body plan, while others, such as starfish and lots of other sea-living creatures, have radial symmetry. These organisms can be divided in half along any one of a number of lines, so long as they include a central point, and the two sides would still be symmetrical.