By Vidya Rajan, Columnist, The Times
To observe a graceful animal in motion is a beautiful thing.
This is even more so for those justly celebrated for their mastery of movement, such as a hummingbird hovering over a flower, or a human thundering to a heart-stopping sub-10 second 100 meter Olympic run. The seeming perfection of form is an illusion – all forms are in development, with each generation adding and pruning visible and invisible structures that make the animal better adapted to its environment in order to succeed in the struggle for life.
Charles Darwin immortalized this concept in the last sentence of his book, On the Origin of Species by Means of Natural Selection. He writes, “Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
In this, the second article in the “Endless Forms most Beautiful” series, I will examine the evolution of body forms “from so simple a beginning”, as Darwin so eloquently puts it. In fact, the evolution of sequential developmental stages is so well established, with so much evidentiary support, that there is a whole biology field called “EvoDevo” or Evolutionary Development. This article is a short introduction to the field. But, where do we start? What is the simplest form in animals?
The answer will come as a shock. The simplest animal is a sponge, and it is the basal animal from which all other animals evolved. That makes it our earliest animal ancestor. But how can this be? A sponge has no muscles and does not move. How does this blob make it into our family genealogy? There were five sequential structural changes to the primitive sponge body plans that gave rise to the most advanced and complex animals are: 1. Multicellularity, 2. Tissues (which goes hand-in-hand with symmetry), 3. Presence of a coelom, which is a cavity in the body, 4. Developmental fate of the blastopore, an opening in the early embryo, and 5. Segmentation of the body into independent sections, some of which evolved limbs with joints.
I’m sorry for the lecturing tone of this article. It’s difficult to present these quite disjointed factors in a single, smooth narrative. Bear with me, and hopefully it will all make sense at the end.
The transition from unicellularity to multicellularity was the first of several pivotal events in animal evolutionary history [1]. Although it happened once in animals, it occurs frequently in slime molds and some fungi which transition normally between unicellular and multicellular forms. In animals, this event in a protist organism called choanoflagellate is thought to have eventually led to all multicellular animals. There is an even more interesting parallel observation. When a fertilized egg develops, its earliest stages resemble an amphiblastula, a sponge larva, across all animals (Figure 1) . This recalls the famous saying “Ontogeny recapitulates phylogeny” that anyone who’s ever taken a college biology course knows. This statement means that the embryonic stages of an animal show all the stages of development of its evolutionary ancestors. This is fun to actually examine, but is beyond the scope of this article. By the way and speaking of animals, one of the strangest things about sponges is that they are a collection of cells, not an integrated body. If a sponge is strained through a sieve, the separated cells will re-aggregate to form a single body. That is one strange animal.
Figure 1: Comparison of the developmental stages of a sponge, jellyfish, and mouse. The sponge is an aggregation of cells, rather than an individual, but its amphiblastula larvae bears a striking resemblance to stages during development of primitive (jellyfish) and advanced (mouse) animals. Figures are not to scale. The abbreviations in development of a mouse figure are: ICM: inner cell mass, TE: trophoectoderm, AVE: anterior visceral ectoderm, AME: anterior axial mesendoderm.
Figures modified under Creative Commons license from devbio.biology.gatech.edu (mouse), https://en.wikisource.org/w/index.php?title=Page:EB1911_-_Volume_25.djvu/750&oldid=8874694 (sponge),https://upload.wikimedia.org/wikipedia/commons/5/5c/La_morphologie_et_stades_developementaux_de_Nematostella.jpg (jellyfish)
The next key stage of development of complexity was the formation tissue layers. Whereas sponges have no tissues, the primitive jellyfish and relatives have two layers: an outer layer of skin and an inner layer of cells containing muscles that could propel the animal. Jellyfish also have radial symmetry, with no head or tail although they have an upper and lower side.
Subsequently, the development of three tissue layers by a process called gastrulation had two outcomes: 1. It caused the blobbish, round structure that was the jellyfish form to elongate, producing an animal with a bilateral symmetry and three axes: head/tail, back/front, left/right. 2. The three layers could follow independent developmental trajectories – the outer layer makes the skin, gonads, and nervous system, the middle layers formed the musculo-skeletal and circulatory system, and the inner layer makes the digestive tube and all its supporting organs. Though jellyfish could only leverage their two layers so far, having three layers allowed the middle layer, the filling between the essential outer and inner layers to develop a huge amount of complexity without imperilling the survival of the organism. The three layers pattern also allowed the formation of a tubular organism with a head and tail and a tube-within-a tube structure (Figure 2).
Figure 2: Schematic of a triploblastic organism with bilateral symmetry. See text for details. Figure modified from https://en.wikipedia.org/wiki/Bilateria#/media/File:Bilaterian_body_plan.svg under a Creative Commons License.
The third big development was the formation of a coelom. The coelom is a curious thing. It is actually the cavities in your body which house your organs. Imagine being in a surgical theater: a heart bypass would lead to cutting into the thoracic cavity, a surgery for appendicitis would cut into the abdominal cavity. Both are derived from the coelom. Coeloms are lined on each side by the middle layer of tissue which can develop into muscles, blood, cartilage and bone. The coelom did not develop all at once! Flatworms have no coelom – thus they are flat. All the three layers of the body are stuck together. In the next iteration, a cavity develops within the middle layer, such that the organism becomes round, not flat (Figure 3). To visualize the effect on the body, imagine a firehose with no water in it which flops around, and compare it to a firehose which is full of water. The unfilled firehouse is floppy, and the water-filled firehose is rigid and is therefore a better body plan because it can push through objects and provide resistance for muscles to push against. All advanced animals that have rigidity, from earthworms to mammals, have a coelom.
Figure 3: Coelom. The coelom is a cavity lined on both sides by mesoderm. The cavity provides the resistance for muscles of the mesoderm to push against for movement. Modified under a Creative Commons License from https://en.wikipedia.org/wiki/Coelom#/media/File:Figure_27_02_05.jpg
For the next aha moment in evolution, recall the embryonic stage of the sponge, the amphiblastula, that all animals pass through. In more advanced animals, this stage is called a “blastula” and resembles a beach ball with a thin layer of cells forming the ball and it is hollow within. Subsequently during a process called gastrulation, it collapses into itself, leaving only a small external hole that widens into a cavity (Figure 4). This process produces the layers of the body, and the cavity becomes the gut. Not only that, the hole is all-important in developmental terms. If the hole becomes a mouth, then the organism is protostome and develops into an invertebrate. If the hole becomes an anus, this organism is a deuterostome, and becomes a vertebrate such as humans, or a starfish. Invertebrates have an exoskeleton, and they have determinate cell fates, meaning that if you take a two-celled embryo and separate the cells, one will develop into a head and the other into a tail. Invertebrates include worms, snails, and insects. By contrast, vertebrates have endoskeletons, or skeletons inside the body. They also have indeterminate early embryonic stages. Separate two cells at an early stage of development, and they will each develop into a whole organism. This is true for all deuterostomes, from fish to amphibians to birds to mammals. This is why you will never see a bee larva with a twin derived from the same egg inside the same cell, but you do get human identical twins. Remember, bees are protostomes, where the blastopore develops into a mouth. Bees also have exoskeletons made of chitin, not bones inside their bodies made rigid by calcification.
Figure 4: The process of gastrulation. Gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. A gastrula has 3 germ layers–the ectoderm, the mesoderm, and the endoderm. Some of the ectoderm cells from the blastula collapse inward and form the endoderm. The blastospore is the hole created in this action. Whether this blastospore develops into a mouth or an anus determines whether the organism is a protostome or a dueterostome. This diagram is color coded. Ectoderm, blue. Endoderm, green. Blastocoel (the yolk sack), yellow. Archenteron (the gut), purple. Figure by Abigail Pyne modified under a Creative Commons License from https://en.wikipedia.org/wiki/Gastrulation#/media/File:Blastula.png
A significant developmental jump was the evolution of segments. The genius of this step is that each segment could develop independently, effectively specializing in function. In the past, I have compared segmentation to inner walls in a house which allows each room to be used for a different purpose. You can observe the progression between primitive unsegmented animals that are basically elongated bags, to the sophistication of segmented animals such as insects and ourselves. For example, segmentation allows insects to develop a huge diversity of mouthparts – those that can suck (bees), cut (horseflies), chew (grasshoppers), and pierce (mosquitoes), all from the same progenitors, while maintaining similarity in other segmental structures such as antennae and legs. Segmentation allows “ripped, cut, swole or jacked” humans to strut their sixpacks. Further, segments allow limbs to emerge only in some segments of the body, and not in others. [Note: Segmentation is controlled by a group of genes called “homeobox” or “Hox” genes and is very well understood [2].] In some animals the additional introduction of joints to limbs facilitates movement. Insects and humans have jointed limbs, earthworms have limbs that have no joints. Joints allow a more springlike motion and thereby greater propulsion, excellent for both catching prey and escaping a predator. Joints have been modified to provide rotational, hinging and gliding motions, and the limbs of both vertebrates and invertebrates have been modified for running, jumping, swimming and flying.
That is not all – vertebrate animals went on to develop limbs that were variously modified into fins, wings or legs, swim bladders that were modified to lungs, embryos that were kept moist inside eggs that could be deposited outside the body or hatched inside the body and protected until they were big enough to care for themselves. Some animals also nourish their young for a long time post-birth with milk and care for them until they are able to fend for themselves. Whereas many animals have a social structure that cares for their young, few have a structure that care for their weak that we know of, except mammals. Whales and dolphins have been observed tending to injured and sick mates, as do rats. Humans are not that special in this regard. Based on some awe-inspiring evolutionary research by Neil Shubin and his colleagues at the University of Chicago, PBS has developed a great resource for exploring your inner animal. It’s at http://www.pbs.org/your-inner-fish/interactives/explore-your-inner-animals/ and it’s a whole lot of fun!
Finally, evolution nerds who fret about these things wonder if the first animal was a jellyfish rather than a sponge. Whether sponge or jellyfish, the first animals were not exactly hummingbirds or Olympic athletes. What is so special about humans is that we are probably the only species that reflects on our origins to the extent that we do. And may we never stop wondering.
- Grosberg, R.K. and R.R. Strathmann, The evolution of multicellularity: a minor major transition? Annu. Rev. Ecol. Evol. Syst., 2007. 38: p. 621-654.
- Arendt, D., Hox genes and body segmentation. Science, 2018. 361(6409): p. 1310-1311.
