A Step-by-Step Guide to Animal Evolution

A Step-by-Step Guide to Animal Evolution

All life on Earth is connected. Whether you're a marine worm or a marmoset, the same genetic code proliferates your DNA. Join me on an overview of animal evolution and see the major transitions that created all animal life today.

Life on Earth began almost as soon as the environment could support it. You can trace the story through DNA, which exposes a common ancestry among all life that dates back 3.8 billion years.

But how can dinosaurs and dandelions be related? How can human intelligence have arisen from brainless bacteria?

The answer is evolution. The blind, trial-and-error process that stumbles on incremental adaptations and—over massive periods of time—builds them into the breathtaking diversity of life.

What is Evolution?

Evolution by natural selection is a beautiful theory, but it can take multiple exposures to get your head around it properly. In a sentence, Darwinian evolution is the emergence of new species through random genetic mutations. Naturally, that sentence should beget many questions which I hope to answer today.

It's a wild thought. Evolution is fuelled by mistakes in DNA replication. But how could the complexity of human beings have arisen out of such random chance?

Mutations create genetic diversity, which in turn provide multiple solutions to survival. It means that some individuals within a population are better adapted to survive and reproduce than others.

Darwin and Wallace's evolution by natural selection explains how such random mutations can have either positive, negative, or neutral effects on survival. In other words, not all mutations are helpful, and they certainly aren't goal-oriented. Hence the title of the Richard Dawkins classic, The Blind Watchmaker.

Some mutations cause diseases that mess up your chances of survival altogether. Others are completely benign because they don't code for anything at all.

But sometimes—relatively rarely, in fact—a mutation gives an individual a competitive edge.

The Hump Advantage

Take the mutant camel who has a sightly bigger hump than the standard camel. He can store more fat in his hump, meaning he can survive for longer periods of time without having to expend energy on feeding.

He now has better chances of survival than the standard camel, and so has better chances of reproducing and passing on his mutant hump gene.

That may sound trivial if you consider one camel at a time. But we're looking at the incremental effects of mutant genes arborising through a population over many generations.

It all starts with one camel, but over multiple generations where big humps benefit survival, all camels in the population end up with the mutant gene.

Camel Evolution

Survival of The Fittest

Now take a mutant beetle who has a pigment that gives him more effective camouflage than the rest of his colony. He's now less likely to get picked off by a sharp-eyed predator.

He survives far longer than the typical beetle and has more opportunities to mate and spread his mutant genes. Over many generations, the camouflage mutation spreads through entire beetle populations and the species as a whole acquires a new trait.

Beetle Camouflage Evolution

Mother Nature is a bitch. Over time, the weakest individuals in a population (say, with the least effective camouflage in this instance) fail to grow up and reproduce, and their genetic traits die out with them.

What creates the incredible diversity of life is that there's no single solution to survival on planet Earth. The beetle's need for camouflage changes entirely depending on his environment.

It might be bright green if he lives in the jungle, or dusty yellow if he lives in the desert. Earth's environment is forever changing, so as long as life is able to mutate, it can continue to evolve to meet the demands of an evolving competition.

This process has shaped the way all life on Earth has survived and thrived for billions of years, never settling on a single ideal form because of the many environmental habitats and ever-changing pressures within them.

You end up with a planet full of different species, each adapted extremely well to their current environmental niche.

The Three Domains of Life

Life on Earth is categorised into three domains: bacteria, archaea, and eukarya. Molecular analysis of DNA tells us these domains arose a stupidly long ago from a common single-celled ancestor, whose primitive genome can be inferred from the life that exists on Earth today.

Three Domains of Life - Bacteria, Archaea, Eukarya

While much evolutionary research is done on bacteria (because they spawn a new generation every 20 minutes) this post is a step-by-step guide to the more familiar animal evolution.

So let's fast-forward through the first few billions of years of evolution involving only single-celled bacteria and archaea. We start our journey with the first multi-celled animal. And it's probably not what you think.

1. The Evolution of Multi-Cellular Life

Animals evolved 770 million years ago when an ancient single-celled organism called a choanoflagellate began living in colonies.

Over many generations, this adaptation to stick together saw the rise of new simple organisms called Porifera, or sponges to you and me.

You may not think of sponges as animals. They have no brains, eyes, blood, organs, or even true tissues. They are merely clumps of eukaryotic cells which have specialised to function in complementary ways.

Porifera - Sponges

Scientists first recognised sponges were the first animals by the fact that single-celled, free-living choanoflagellates are structurally indistinguishable from the body cells of sponges alive today.

More recently, genetic analysis has verified that they share a ton of common genes with more complex animals.

Roald Dahl had it right when he named Aunt Sponge. This is your most distant animal relative in the world. Sponges are your cousins, albeit a gazillion times removed.

2. The Evolution of Radial Symmetry

Jump forward 100 million years after the emergence of the sponge. (We're still talking 670 million years ago, if you can imagine such a time period. Which of course you can't.)

The next major developments in animals began to emerge from cumulative adaptations. Soft bodies and radially symmetrical forms gave way to basal species of jellyfish and corals.

Cnidaria - Jellyfish

Their radial symmetry gave them bodies which have a top and bottom, but no left or right sides. Like a pie, they can be sliced up equally along many axes and still look the same.

This body plan develops extremely early in embryonic development and it's an important way for biologists to distinguish various stages of animal evolution.

For the first time on Earth, animals now boasted distinct tissue layers. You would boast about that too, I believe. They also developed an internal body cavity for digesting food and transporting nutrients.

Now that's some fancy biological equipment.

3. The Evolution of Bilateral Symmetry

Aquatic animals eventually diversified into many exotic forms.

Flatworms evolved some 550 million years ago. They had a new feature called bilateral symmetry: having a distinct head-end (cephalisation) with mirrored right and left sides. As humans we are also bilaterally symmetrical.

The significance of this may not be clear until you consider the lifestyles of radial vs bilateral animals.

Radial animals are usually immobile (think sea anemones) or drifters (think jellyfish). Their all-round symmetry means they can meet their environment equally well from all sides: for attack, defence and mating.

Bilateral animals are active movers, encountering their environment head-on where all the sensory equipment is located.

The flatworm also possesses a central nervous system, with bundles of nerves (ganglia) concentrated in its head. This is an early—if primitive—brain.

Platyhelminthes - Flatworms, Flukes

4. The Evolution of Exoskeletons

If you were to go outside and pick up a handful of soil you would encounter thousands of our next guest: Nematoda.

These tiny squiggly guys are equally content living underground, in the ocean, or in your gut. Their evolutionary super power is a tough outer coating (a cuticle) which serves as an exoskeleton to protect their inner organs.

They're simple, yet highly adaptive creatures, and that's precisely what makes them so successful.

Nematoda - Roundworms

Next, the animal phylum Arthropoda includes a million species of insects, spiders and crustaceans which account for 80% of all animals on Earth. In other words, arthropods rule.

Like nematodes, they feature an exoskeleton which they must moult and re-secrete to grow into their adult form. They also have jointed limbs and segmented bodies, both of which can be highly specialised.

For instance, a lobster's appendages are specialised into various forms: sensory antennae, pincers, mouth-parts, walking legs and swimming appendages. Different arthropod species have different combinations of appendages suited perfectly to their environment.

Arthropoda - Insects, Spiders, Crustaceans

5. The Cambrian Explosion

Like many of today's living animal phyla, Arthropoda first appeared during the Cambrian explosion, a time of relatively rapid evolutionary change 525-535 million years ago.

This is a fascinating period of geological and evolutionary history. Scientists can relate massive environmental changes over millions of years to the explosion of new lifeforms on Earth.

The Cambrian Explosion is a wild demonstration of evolution by natural selection.

But what caused the Cambrian explosion?

Environmental Factors

Over time, the global environment was changing. Earth saw increased oxygen levels and massively receding glaciers.

An essential fuel for metabolism, more oxygen meant animals could grow larger and pursue more energy-intensive lifestyles, such as hunting.

Meanwhile, a fall in glaciation meant more light could penetrate the oceans, and as you well know, sunlight was essential for the aquatic plants to thrive and become food.

Ecological Factors

Relatively rapid co-evolution of various animals pushed predators and prey into a race for survival.

For instance, predators developed adaptations to swim faster, while prey evolved hard protective shells to make them less susceptible to being eaten.

Genetic Factors

Animal Hox genes underwent key refinements at this time. These are the master control switches for body plan development, so that small mutations in Hox genes can create massive differences in physiology.

In a relatively short space of time, Hox mutations saw many developmental variations tested out by nature.

With the Cambrian explosion in mind, let's explore some more major animal phyla and their novel features that evolved half a billion years ago.

6. The Evolution of Body Segments

Back in wormsville, Annelida evolved, including the deliciously slimy earthworm. Named for their segmentation, each body chunk is internally and externally identical to the next.

Some annelids have paddle-like feet which also function as gills. Why gills, you cry? Many annelids are actually marine critters, drifting in the ocean or burrowing into the sea floor.

Some annelids are tiny: less than a millimetre. Others are behemoths: consider the Giant Australian Earthworm which can top three metres. Others suck your blood, like leeches.

Annelida - Earthworms

7. The Evolution of Shells

Sometimes when you walk along the beach you find a really nice shell. These are not naturally occurring fancies but rather the calcium carbonate shells of Mollusca.

Despite first appearances, molluscs are actually a really interesting and diverse phylum. Their special features include soft bodies which excrete a hard protective shell.

Most molluscs are marine (clams, oysters, snails and squid) although some live in freshwater or on land.

Mollusca - Snails, Clams, Octopuses

Wait a minute, Columbo, did you say squid?

Yes—squid are molluscs too. They have a small ancestral shell (a gladius) which is internal. It supports the squid's mantle and serves as a point for muscle attachment.

Take a moment Google Cephalopods which is the collective name for octopuses, squid and chambered nautiluses. Never mind, I'll do it for you. Aren't they brilliant?

These intelligent invertebrates independently evolved eyes and, in the case of colossal squids, grow up to 14 metres in length. They are truly incredible creatures.

8. Evolution in Embryonic Development

Did you know that you have quite a lot in common with starfish?

DNA comparison reveals that sea stars, sea urchins and other Echinodermata are sisters to all living chordates like you and me.

Chordates—along the lines of vertebrates—are a group we'll cover exclusively from here on in our animal evolution.

The striking similarity between a starfish and yourself is your deuterostome mode of embryonic development.

Back when you were eight cells small inside your mother's Fallopian tubes, you took on a special mode of indeterminate cell division. This means that if we isolated a single cell from your embryo at this early stage of life, it could go on to divide and form a complete human being.

This, incidentally, is how identical twins are possible, and where stem cells are in abundance.

The alternative to deuterostome is protostome development, and was the original form of embryonic development, as seen in all animal phyla thus far. Here, the fate of all embryonic cells are sealed early on.

Besides their breakthrough evolution of deuterostome development, echinoderms boast unique features, such as an internal water canal system branching into tube feet which together enable movement and feeding.

Echinodermata - Starfish, Sea Stars, Urchins

Like sponges, sea stars may not look like animals, but they are very much living creatures which breathe, eat and reproduce, either sexually (with sperm and egg) or asexually (breaking in two and regenerating).

8. The Evolution of Chordates

We now move into the evolutionary territory of chordates which includes everything from dinosaurs to dingos.

Chordates are animals with four key features:

  • A nerve chord which develops into the brain and spinal chord
  • A flexible notochord which is the start of a backbone
  • Pharyngeal slits which function as gills or inner ears depending on your species
  • A post-anal tail typically for swimming, though a quick feel of your rear will confirm it's much reduced in humans

The earliest chordates were strikingly like previous invertebrates.

Lancelets, for example, were little blade-like critters which burrow backwards into the sand, leaving just their mouthparts exposed to catch passing food particles.

Cephalochordata - Lampreys

Another example is the sea squirt, or Urochodata. You may also know them as tunicates.

Urochordata - Tunicate

The larval form, which may last only a few minutes, is a tadpole-like creature, swimming in search of a substrate on which to attach itself.

Like you and I, it has all the chordate features. But then it undergoes a radical metamorphosis. The sea squirt re-absorbs its own brain.

Major anatomical features—including the tail, notochord and a primitive eye—are dissolved back into the body. It spends the rest of its life as an immobile little squirt, siphoning water through its body to eat.

After that, the complexity of chordates really stepped up a gear.

9. The Evolution of Vertebrates

Having glimpsed some of the more alien-like animals throughout evolution, we now move on to the more familiar.

The next major development is represented by Myxini, or the hagfish: a jawless fellow with a skull made of cartilage.

Take a look at this beauty queen.

Myxini - Hagfish

The hagfish is a slime-producing marine animal that, while incredibly ancient, still exists as a number of different species today. (Remember that the evolutionary tree has many branches and is no straight line.)

It has rudimentary vertebrae, a brain, eyes and other sensory organs. In evolutionary terms, he's kind of a big deal.

Our next guest is also a face for radio.

Petromyzontida - Lampreys

Lampreys, or Petromyzontida, are other early adopters of backbones. This is your great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-grandmother. You really have her eyes / tail / sucker mouth.

10. The Evolution of Jaws

Next came the evolution of jaws and a mineralised skeleton. Welcome to the age of true predators.

Early Chondrichthyes (like sharks and rays) were able to eat big chunks of flesh, so grew bigger and swam faster compared to their ancestors.

With such exotic and fast-moving predators emerging, evolution was racing along.

Chondrichthyes - Sharks, Rays, Skates

11. The Evolution of Lungs

You may have noticed that we're still largely in the ocean along our evolutionary trail. That's because sea animals had not developed any capacity to gulp air. Until now.

The ray-finned fish, Actinopterygii, diverged from its ancestral line carrying the unique trait of maneuverable fins and a swim bladder.

For many fish, the swim bladder is a buoyancy aid; an air-filled sac which keeps them at their water depth without having to waste energy on swimming. It is also a rudimentary lung.

Actinopterygii - Ray Finned Fish

We can't leave fishy territory without acknowledging the lobe-fins, or Sarcopterygii. The most famous of which is the coelocanth, thought to have gone extinct along with the dinosaurs 65 million years ago.

In fact, they were discovered to be alive and well when, in 1938, a fisherman hauled one in and took it to local naturalist:

"I picked away at the layers of slime to reveal the most beautiful fish I had ever seen. It was five feet long, a pale mauvy blue with faint flecks of whitish spots; it had an iridescent silver-blue-green sheen all over. It was covered in hard scales, and it had four limb-like fins and a strange puppy dog tail."

Marjorie Courtenay-Latimer

Sarcopterygii - Lobe-Finned Fish

12. The Evolution of Tetrapods

By now, the most complex life forms on Earth had eyes, a brain, a backbone, jaws, gills, lungs and muscular fins. Only relatively small modifications of this body plan were needed to evolve into tetrapods ("four feet").

The 2006 discovery of a species called Tiktaalik provided the so-called missing link between the aquatic lobe-fins and land-dwelling tetrapod animals.

Tiktaalik: The Missing Link

Like a fish, Tiktaalik had fins, gills and lungs, and its body was covered in scales. But unlike a fish, it had ribs to ventilate its lungs and muscular fins to support its body out of water. It also had a neck and shoulders to help move its head.

Perhaps most significantly, Tiktaalik's fin-feet had the bone structure common to tetrapod wrists today. This extraordinary creature would give rise to the first tetrapod land dwellers: Amphibia.

Amphibia - Salamanders, Frogs, Caecilians

While amphibians are comfortable on land, their lives are inextricably connected to the water. Their moist skin and eggs are vulnerable to drying out, so they can never truly explore the far reaches of dry land.

Nonetheless, they've been a successful group of animals which includes all frogs, salamanders, and caecilians (legless snakes).

They go through quite the metamorphosis early in life, having larval forms which are distinctly different from their adult forms.

13. The Evolution of Amniotic Eggs

If gooey eggs tied early amphibians to the water, then hard-shelled eggs would liberate their descendants: the massively diverse phylum of Reptilia.

Reptilia - Snakes, Lizards, Crocodiles, Birds

We're going to spend a disproportionate amount of time looking at reptiles because they include some of the most popular animals that ever existed: dinosaurs, birds, snakes, lizards, crocodiles and turtles.

Reptilia evolved over millions of years, of course, and some are much more closely related than others. What might surprise you is that birds are indeed reptiles and their closest surviving relatives today are the crocodiles.

Consider Testudines, the bizarre shelled-reptiles known as turtles, tortoises and terrapins.

These are seriously ancient lizards: the oldest sea turtle fossil dates back 120 million years. It lived alongside dinosaurs and its body plan remains virtually unchanged today.

Testudines - Turtles, Tortoises, Terrapins

Dinosaurs are a completely different evolutionary branch among Reptilia and lived between 230 and 65 million years ago, a time collectively known as the Mesozoic Era.

We can split dinosaurs into two major groups.

First up, Ornithischia (meaning bird-hipped) were mostly herbivores named for a pelvic structure similar to that of birds.

Over the course of evolution, some ornithischians developed armoured plates and thick skulls which protected them from predation by carnivorous dinosaurs.

Ornithischia - Ankylosaurus, Parasaurolophus

The other group of dinosaurs were the Saurischia (meaning lizard-hipped) which included beastly fellows like T-Rex and Giganotosaurus; massive docile long-necks like Diplodocus and Brachiosaurus; and smaller feathered critters like Archaeopteryx.

Saurischia - T-Rex, Diplodocus, Archaeopteryx

For a long time, Aves, or birds, were ironically thought to have descended from lizard-hipped dinosaurs. However recent research suggests we need to shuffle our evolutionary conception. They probably descended from bird-hipped dinosaurs after all.

Aves - Blackbirds, Ducks, Parrots, Keas

What we do know for sure is that birds evolved some remarkable bodily adaptations to aid flight. Hollow bones, lack of teeth, a single ovary in females, and specialised feathers all help to get birds off the ground.

This has massive survival benefits in terms of hunting, escaping predators, and widening their territories.

In contrast, take a look at native New Zealand birds, who evolved in geographical isolation for 80 million years and with no mammal predators.

Many species, such as the kiwi and kakapo, evolved to become flightless. So when 19th-century Europeans arrived with cats, rats and dogs, flightless bird populations crashed. It demonstrates how flight is an incredible survival adaptation.

14. The Evolution of Mammary Glands

We have now briefly considered all modern day animal phyla except one: Mammalia. By definition, mammals are animals which have hair and produce milk from mammary glands. I don't know my dad's excuse.

Fossil evidence shows how the jaw was gradually remodelled in early pseudo-mammals, over a period of 100 million years, and alongside the dinosaurs.

By the end of the Triassic period, mammals were mostly small, hairy creatures which fed on insects at night and probably still laid eggs.

They diversified to a degree, but the competition was fierce: dinosaurs already dominated many ecological niches.

Three lineages of mammals—monotremes (egg-layers), marsupials (with pouches) and eutherians (with placentas)—were already established when many dinosaurs went extinct due to major environmental shifts, probably involving a very big meteor.

The mammals that survived exploited the ex-dinosaur habitats, food sources and territories, rapidly filling the ecological niches left behind.

This was our chance to shine.

Mammalia - Monkeys, Rhinos, Llamas, Bears

Over the next 65 million years, mammals diversified into various forms and became another successful phylum. Especially if you count the spread and impact of mankind.

This brings us to the present day. Congratulations for getting this far, both in terms of your animal evolution and reading this article. You, sir, are fit to survive.

As you can see: from choanoflagellates to chameleons, life finds a way.

Yes, I'm quoting Jurassic Park. There's no circumstance in which it's not appropriate to quote Jurassic Park, but in this case it's extra relevant. Jurassic Park was a sharp-toothed demonstration of how animals will ruthlessly exploit their environment to survive.

Read the actual book if you haven't already (or read it again if you have). It's really good.

In fact, the majority of life that has ever existed on Earth has struggled and fought not to die young. Many perished in the jaws of some larger animal or at the mercy of famine or disease or catastrophic environmental change.

These are the challenges which shaped the evolution of all animals ever, including humans.

Until now.

Are We Still Evolving?

The famed zoologist David Attenborough suggests that, for the first brief time in billions of years, we may have stopped evolving.

Of course, we still make mistakes in our DNA replication, and this still creates novel mutations. And let's not forget that evolution takes a heck of a long time to ripple through generations and therefore populations.

But take a snapshot of this moment in time and we can observe our species wielding considerable control over our environment. The effect of evolution by natural selection is then significantly diminished.

Agriculture ensures a steady food supply. Vaccines ensure herd immunity against infectious diseases. Criminal justice ensures the weak aren't preyed upon by the strong.

In the near future, gene editing and other molecular therapies will offer a direct method of altering our genetic code, paving the way for permanently altering our germ lines.

As long as we thrive this way, Mother Nature can no longer kick us out of the gene pool. Instead, we swim to our hearts' content, few of us pausing to realise that billions of animals have lived and died at the mercy of evolution.

Becky Casale Bio

ABOUT THE AUTHOR: Becky Casale is a science freak and a writer who pooped out Science Me. She is studying for a BSc and writing her first sci-fi novel.