Why our bodies are symmetrical, but only on the outside – 09/30/2024 – Science

When observing the incredible beauty and diversity of life on Earth, it is clear that almost the entire animal kingdom shares a common characteristic: bilateral symmetry.

From butterflies to walruses, from Tyrannosaurus to Homo sapiensmost animals have a right side and a left side.

If we reflect the right half of an animal in a mirror, we will see that it is almost identical to the left half. Why is this form so common?

To understand, we need to travel to the depths of the ocean… and to the remote past.

We are going back 570 million years, to the Ediacaran period, when animal life existed only in the oceans.

If we dived, we would see “a kind of forest covering the ocean floor, with strange leaves floating, probably translucent or gray, up to a meter high”, describes Frankie Dunn, a paleobiologist at the Natural History Museum at the University of Oxford, in the United Kingdom. United.

These strange leaves, she says, “are the oldest things that we can safely say are animals.”

Examples include charnia.

“It looks like a plant, but we know it’s an animal because it grows in the same way as animals, excluding any other possibility,” explains Dunn.

“Furthermore, as it lived at depths where light did not reach, it could not carry out photosynthesis. At first glance, it may appear bilaterally symmetric, but the branches develop sequentially, what we call gliding reflection symmetry, characteristic of many organisms in the world. Ediacaran period.”

We call it this way because it’s like you cut a symmetrical pattern in half and slid one side up slightly.

Currently, we have not found anything similar to these charnias.

They existed at a time when the first animals experimented with different unusual body shapes, as if life was testing different “clothes” until it found one that really worked.

“There were many different forms of symmetry at that time, some of which have disappeared, but which may have been very useful during the Ediacaran, as the world was very distinct and organisms responded to different environmental pressures”, notes the expert.

This period of great symmetrical diversity did not last forever.

Eventually, worm-like creatures began to emerge, whose shape — with head and tail — revolutionized everything.

Bilateral dominance made locomotion easier: “If you have a mouth on one end and an anus on the other, you can move much more easily, since you’re not expelling waste as you move,” explains Dunn.

“With bilateral symmetry, you can be more streamlined along your main body axis, organize muscles along the extremities of your body, and concentrate sensory structures at one extremity, allowing for the diversification of complex behaviors. Animals began to burrow into the sediment, swim and explore the world in three dimensions.”

It’s hard to underestimate how much bilateral symmetry has changed the rules of the game.

A digestive tract, where food enters at one end and exits at the other, offers a natural direction of movement which, simply put, is toward food and away from waste.

Animals like these Ediacaran worms moved much better than other life forms.

Thus, competition for food sources became much more intense. Bilateral animals surpassed all others, and, with their success, they changed the environment so much that they redesigned the planet.

Dunn notes that “the animals that inhabited the Ediacaran seabed had different symmetries. They changed the world completely when they began to interact with the microbial soil, which had little oxygen. As they penetrated it, they oxygenated it and began to destroy the environment where other creatures lived, condemning them to extinction.”

The appearance and diversification of animals with bilateral symmetry is a profound turning point in the history of life on Earth.

But why do almost all animals have bilateral symmetry?

Because this body design proved so effective that, once it emerged 570 million years ago, it became an enduring success, dominating to this day.

Bilateral symmetry is the mold of animals. Well, almost all animals.

Strange exceptions

One group of animals that defies this rule of bilateral symmetry are the echinoderms, which include sea stars, sea urchins, sea cucumbers, and brittle stars.

“They are very different in their body plan and design, and can teach us a lot about evolution and its limits,” points out Imran Rahman, lead researcher at the Natural History Museum in London.

Rahman confesses: “I’ve always been fascinated by rare animals, sometimes called strange wonders.”

And with good reason: they are surprising and intriguing.

The starfish, for example, begins its life as a larva with bilateral symmetry.

“Then, in metamorphosis, the adult grows on one side, while the other side disappears.”

Once adults, these pentaradially symmetric stars look like they were made to grace the sea floor, but where are their heads? “That’s a subject of debate,” says Rahman.

“Recent research suggests that almost the entire animal is the head, without the posterior end that we see in other animals. So a starfish would be a kind of disembodied head crawling over its lips.”

Despite their distinct bodies, echinoderms have existed for hundreds of millions of years. Why did they survive in a world dominated by animals with bilateral symmetry? Nobody knows for sure.

And the plants?

Being bilateral was a recipe for success for animals, but in plants the panorama is more varied.

“To understand why plants don’t appear symmetrical, you need to observe their development,” says botanist Sophie Nadeau, from Paris Saclay University, in France.

“In animals, the body plan is definitive: when you are an adult, you don’t grow anymore. Plants are made up of modules (stem, leaves, flowers). Just stack these modules and you get a plant that can grow indefinitely.”

If a plant grew under perfectly controlled conditions, it would probably be quite symmetrical, adds Nadeau.

But the truth is that they do not grow in isolation; winds, sunlight and available space influence its growth.

“Sometimes one part develops more than the other, resulting in an architecture that is not perfectly regular.”

However, even though they are rarely perfectly symmetrical, plants exhibit symmetry in several aspects: their leaves, for example, often have bilateral symmetry.

“If we look at each organ, they have symmetry. Stems have almost perfect radial symmetry. If you cut the trunk of a tree, it has radial symmetry. So each organ is actually symmetrical.”

Thus, it is possible to find symmetry in plants, depending on where you look… even internally.

And this is interesting, because if we go back to animals, the situation is opposite. Despite our external symmetry, internally things are much less symmetrical.

Here and there

“There are many fascinating asymmetries in the human body,” notes Professor Mike Levin, from Tufts University in Massachusetts, USA.

“Some are anatomical, such as organs such as the heart, stomach and liver, which in normal individuals are located on only one side of the body. Other, less obvious asymmetries occur in the brain, which is slightly different from one side to the other . There are also interesting functional or physiological asymmetries; for example, certain diseases occur more frequently on one side of the body, such as breast cancer, which tends to be more common on one side.”

“These hidden asymmetries reveal that cells actually know they are not equal.”

The reason for this asymmetry in our internal organs, with the liver or spleen on one side and our intestines curled up behind, may simply be the most efficient way to organize everything.

“But how embryos reliably determine which side of the body should contain the heart, the intestine, etc., is a fascinating question,” comments Levin.

Asymmetry as a puzzle

Asymmetry is an enigma, as it is difficult to achieve for biological systems. You can use gravity to determine your vertical position, but calculating left and right is much more complex. How do cells do this? We really don’t know.

“At what point in embryonic development do cells figure out which side of the body they’re on? If you’re a ball of cells, how do you know where your midline is and what mechanisms allow you to tell left from right?”

Those are a lot of questions. One answer may be related to the way cell molecules self-organize into spirals, creating an asymmetry that is then amplified during development. But regardless of how it occurs, according to Levin, asymmetry can be fundamental to life.

“Asymmetry permeates all of biology, from quantum events that break symmetry to development, behavior and even our works of art. It is surprising how it connects from the subtle molecular properties of the quantum world to its cultural and social impact.”

Although asymmetry remains a mystery, the reasons for symmetry, at least in humans and other bilateral animals, are clear.

It’s a very advantageous design, as Frankie Dunn points out: “Bilaterals were destined to triumph because their body plan is so well suited to many activities, such as flying, swimming and walking, as well as being highly susceptible to innovation.”