Inner Nature: Shapes of animal bodies

By Vidya Rajan, Columnist, The Times

The study of animal form and function is both illuminating and fascinating, panning from simple to complex forms. The morphing started 750-odd million years ago. At this time, the Earth was not young by any means! It was already about 3.5 billion years old (80% of its age today, which is about 4.3 billion years old) and populated by cyanobacterial mats, and single-celled organisms which glommed together to originate multicellularity and division of labor. The multicellular form allowed evolution to test shapes that were successful in the environment and to further adapt the successful shapes. The Cambrian explosion 540 million years ago led to an explosion of diversity of animal shapes and forms including the nightmare Anamalocaris, voracious Opabinia sporting five eyes and an extensible clawed proboscis, the hallucinogenic Hallucigenia and the fish-like precursor to vertebrates, Pikaia.

In this article, I will reprise some of the major changes that led to the diversity we see today, focusing on the sequential modifications which provided the evolutionary success that put humans in a position to study the process of evolution of their own form. I have isolated what are considered the key innovations below. I will illustrate these innovations with videos from a marvelous program called “The Shape of Life”. The videos are fairly long, but informative and beautifully made. You can always speed it up or watch it again to pass the cold winter nights with wonderful stories of evolution of animal form and function.

Figure: An artist’s impression of the oceans following an explosion in animal diversity following the Cambrian explosion. Hovering above is Anamalocaris. Opabinia is catching an unspecified animal. From back to front: Trilobites, Sydneyia, Canadaspis, and two species of sponges.

Tissues: The fundamental innovation was the formation of tissues due to multicellularity, which was basically cells of a similar type sticking to each other after division and then performing the same function. The formation of 2-dimensional tissue sheets allowed the formation of tubes to move material around and allowed the formation of dense 3-dimensional clumps of cells which made up bodies to access nutrients and remove wastes.

Symmetry: The next major innovation was symmetry. Some animals (e.g. sponges) lack any symmetry at all; jellyfish and anemones have radial symmetry like a wheel; and more advanced animals have bilateral symmetry. Bilateral symmetry allowed the body to be drawn out into a long tube and allowed cephalization, which is the formation of a head at one end of the animal and load it with sensors and to better seek prey and then pursue it headlong. One of these earliest predators was an unimpressive flatworm. Unimpressive that is, to anyone who is not its prey. Here is a video of a flatworm innovations. Flatworms not only were the first hunters, but also recorded the earliest sexual jousting.

Coelom: Subsequent morphological innovations were more subtle (and I will omit some technical detail in the interests of clarity, such as it is). The first was the formation of a cavity called the coelom in which animals sequestered organs. There are acoelomate animals (without a coelom) such as the predatory flatworm above, and others with a coelom located oddly between the muscle layer and the gut layer called pseudocoelomates. Acoelomate and pseudocoelomates did not generate structural complexity because their body plan was just not suitable for evolutionary innovation. The tissue origins of coeloms (true coelomates) allowed some animals to grow bones and muscles in between the outer skin and inner gut and includes invertebrates such as octopuses, earthworms and insects, but also all vertebrates including fish, reptiles, amphibians, birds and mammals.

This leads to a “tube within a tube” structure, with the gut padded with connective tissue placed within a body tube. Mammals have several coelomic cavities that you will recognize: the cranial and spinal cavity, thoracic and mediastinal cavities, and the abdominal and inguinal cavities. Much innovation could then occur, including the formation of lungs and digestive organs from the digestive tube, blood, cartilage, muscle and bone in the middle connective layer, and the skin with its sensory organs on the outside. The coelom allows earthworms to dig through soil which is much more dense than its own body using hydrostatic fluid pressure in the coelom to provide rigidity and muscles to power the drilling.

Segmentation: Besides having impressive tunnel-digging skills, annelids like earthworms are segmented. Their bodies are essentially repeated rooms which provides opportunities for each room to be modified to a different purpose. Some contain sense organs, some appendages to move with, and yet others contain different parts of the digestive, respiratory, or immune systems. To see innovation of body form soar, though, we turn to arthropods. Segmented body parts where some segments contained segmented (jointed) limbs allowed arthropods to run, jump, swim and soar to all parts of the planet except Antarctica. As they adapted to land, they evolved hard carapaces to protect from drying out, and impressive organs for breathing and dissecting their food. These animals also metamorphosed from aquatic to aerial forms, a truly great evolutionary innovation. Their activities make life possible. They are the bottom of the food chain, breakdown food, and carry out pollination. Without them, the ecosystem and life on earth will collapse, which accounts for environmentalists’ alarm at the insect apocalypse we are experiencing.

Notochord: Meet the amphioxus, the animal at the base of the tree that contains all vertebrates. While they float unimaginatively head-down in warm seas, their bodies were the prototype that presaged all other vertebrate innovations. Amphioxus itself had many innovations: pharyngeal gill slits that presaged jaws, a tail that protruded past the anus, a hollow nerve cord with cerebrospinal fluid bathing the outside and the inside which allowed brains to grow large, and a notochord. The notochord was a muscular tube that behaved like a spine (because there was no such thing as a spine at the time). Once the notochord evolved, it provided anchorage for muscles that propelled hunters head-first in search of prey. The top predators of the oceans, sharks, have cartilage but no bones, since bones had not yet evolved, and the fearsomeness functionality of the notochord for the predators of its time becomes apparent. Bony fish added minerals to their cartilage, explaining also why young children whose bones have not yet mineralized tend to get bone fractures less than elderly people whose bones are mineralized. Bones provided protection for the brain, spinal cord (which replaced the notochord) and allowed muscles to propel lethal jaws originating from pharyngeal gill slits.

As time went on, limbs arose in some animals from two sets of fins. These “tetrapods” developed bones and weight bearing scaffolding to fight the gravity on land. Chordates on land developed lungs from swim bladders, starting with lungfish. Contrary to common misconception, many fish are quite comfortable on land, thank you very much. Here is a video of some of the fish that walk on land – having amphibious adaptations to support both aquatic and terrestrial landscapes.

Other adaptations such as the ability to lay an egg on land protected from desiccation or to stay warm under a down blanket that you grow or to walk on two limbs while holding your baby with two others and simultaneously scanning the horizon for predators…all these are evolutionary adaptations.

Understanding how form relates to function is impressive in its effect. No one animal or plant is better than another because each is adapted to its environment. As Stevie Wonder and Paul McCartney sang:

Ebony and ivory live together in perfect harmony

side by side on my piano keyboard, oh Lord, why don’t we?

Ebony and ivory live in perfect harmony with plants and animals they evolved on.

Let it be.

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