Why are most of the animals we see symmetrical in appearances (e.g. humans have left-right symmetry) but not plants and trees?


Why are most of the animals we see symmetrical in appearances (e.g. humans have left-right symmetry) but not plants and trees?

In: 8

One of the most basic differences between animals and plants is in their body plan.

Animals (with the exception of the the very most primitive of them) have what you could call a “closed” body plan. They have a set and tightly coordinated genetic program of development by which a fertilized egg divides and differentiates to form a body. This body has a lot of specialization, so it needs to be made the same way each time. If you accidentally leave out the liver, for example, you’re screwed. Many animals have left-right symmetry because they descend from a symmetric common ancestor, but others like jellyfish are radially symmetric instead. Symmetry sort of naturally emerges from development, as the same developmental program gets executed in different directions.

Plants have a different strategy and use an “open” body plan. They are much less specialized in their parts and don’t closely coordinate their entire bodies. Roots go down and shoots go up, but beyond that pretty much everything is under local control with no overall plan. If a part is exposed to sunlight, it makes leaves. If it is under strain, it grows thicker. If it has enough nutrients, it grows longer. Since there is no centrally executed body plan, their appearance depends more on local conditions and life history than anything else. Neat symmetries don’t happen in this way of growing.

Animals are mobile, they can simply move their bodies to where the resources are. Mobility benefits from symmetry, balance and motion are easier to ‘calculate’ if you have matching weights and lengths on either side. So symmetry has a natural advantage and becomes a sexual characteristic in many animals, including humans, which further reinforces the benefits of symmetry.

Plants are not mobile, if they sense nearby resources they have to grow in that direction. Ex a new branch will grow into an opening in the canopy to collect the light from above. It wouldn’t make any evolutionary sense to grow an opposing branch. So plants have no real benefit to symmetry. With one exception: where plants and animals interact, symmetry matters again (flowers).

Just want to add a cool bit here. Not all symmetry is bilateral. Some things have radial symmetry, like sea stars.

In addition to what others have said about how plants versus animals develop, it’s also worth noting the different physiological needs. Plants do not have to “perceive” or move around their environment, judging distances and coordinating limbs. Therefore it isn’t necessary to have eyes/ears on the same plane or a set distance apart, and to have an equal number of appendages of equal length. A tripedal animal with one leg longer than the other 2 and just a single eye would be evolutionarily disadvantaged as both predator and prey.

The vast majority of plants have a TON of symmetry. It’s just on a smaller scale. Flowers are generally super precise and symmetrical, same with a bunch of leaves, grasses, and seeds.