All life on Earth is connected. Whether you’re a marine worm or a marmoset, the exact same genetic code proliferates your DNA. That’s the basis of Evolution 101. The rest of this article explains how we’re all connected by examining the major steps in animal evolution.
Biologists believe life began almost as soon as the environment on Earth could support it. This is backed by our molecular biology (our DNA) which exposes a common ancestry 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?
Ok, concept first. Evolution is the emergence of new species through random genetic mutations. That’s a wild thought: evolution is fuelled by mistakes in DNA replication during sexual reproduction. But how could the complexity of human beings have arisen out of random chance? Ah. I knew you’d ask that.
Mutations create genetic diversity: multiple solutions to living. It means that some individuals within a population are better adapted to survive and reproduce than others. Evolution by natural selection explains how mutations can have either positive, negative, or neutral effects on your survival. In other words, not all mutations are helpful, and they certainly aren’t goal-oriented. Some mutations cause disease and mess up your chances of survival altogether. Others are completely benign because they’re redundant and don’t code for any phenotype (physical attribute) at all. But sometimes – quite rarely – a mutation gives an individual a competitive edge.
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 longer 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 genes. That may sound trivial if you consider one camel at a time, but think about the incremental effects of mutant genes arborising through a population over many generations.
Here’s another example. Take a mutant beetle who has a pigment that gives him slightly more effective camouflage than the rest of his colony. He’s now less likely to get picked off by a sharp-eyed predator. 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. And Earth’s environments are continuously changing, so as long as life is able to mutate, it can continue to evolve to meet the demands of competition in changing environments.
Mother Nature is a bitch. Over time, the weakest individuals in a population fail to survive and reproduce, and their genotypes (genetic traits) die out within a population. 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 own environmental niche.
The Domains of Life
At the broadest scale, all life on Earth is categorised into three domains: bacteria (single-celled organisms that are just about everywhere), archaea (much like bacteria as far as we’re concerned at this stage), and eukarya (multi-celled life including plants, fungi and animals like you and me). Genetic analysis (yay technology!) 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.
Much biological research is done on bacteria, because they have a generation time of about 20 minutes and are extremely useful as experimental models. However, Evolution 101 is going to look exclusively at animals because they’re so much more familiar. So let’s fast-forward through a few billions of years of evolution of bacteria and archaea, starting our journey with the first multi-celled animals.
Animals emerged 770 million years ago when a type of single-celled organism (a choanoflagellate) began living in colonies. Over much many generations, the adaptation to stick together saw the rise of new 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 animal cells which have specialised to function in complementary ways. It is the most basic known animal.
Scientists first recognised sponges were as 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, and sponges are your literal cousins, albeit a gazillion times removed.
Jump forward 100 million years after the emergence of the sponge. 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.
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.
Aquatic animals diversified into many exotic forms. Flatworms evolved from something like hydra (a Cnidarian), some 550 million years ago. These wormy fellas boasted 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. 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; an early, if primitive, brain.
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 squishy inner organs. They’re highly adaptive (albeit simple) creatures and that’s precisely what makes them so successful.
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 secrete anew) in order to grow into their adult form. They also sport 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.
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 history. Scientists can relate massive environmental changes (over millions of years, of course) to the explosion of new lifeforms on Earth. It’s a wild demonstration of evolution by natural selection. But what caused the Cambrian explosion?
Environmental factors point to increased oxygen levels and receding glaciers. An essential fuel for metabolism, more oxygen meant animals could grow larger and pursue more energy-intensive lifestyles, such as hunting. A fall in glaciation also 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 theories suggest co-evolution 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 gobbled up.
The genetic explanation is perhaps the most compelling: “Hox” genes underwent key refinements. Hox genes are the master control switches for body plan development, so that a minor mutation in a Hox gene can create a massive difference in the resulting animal. In a short space of time, Hox mutations saw many developmental variations tested out by nature.
With the Cambrain explosion in mind, let’s explore some more major animal phyla and their accompanying novel features that emerged on Earth some half a billion years ago.
Over 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. Other annelids are behemoths: consider the Giant Australian Earthworm which can top three metres. Others suck your blood, like leeches. Delicious.
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.
Wait a minute, Columbo, did you say squid? Yes, sir, indeed. Squid have a small ancestral shell (a gladius) but it’s reduced and internal. It supports the squid’s mantle and serves as a point for muscle attachment. Now there’s something most people don’t know. Take a moment Google Cephalopods which is the collective name for octopuses, squid and chambered nautiluses. Oh 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.
Did you know that you have quite a lot in common with starfish? Protest all you want, but DNA comparison reveals that sea stars, sea urchins and other Echinodermata are sisters to all living chordates (a group we’ll cover exclusively now along our evolutionary tree of animals). The striking similarity between a starfish and yourself is your deuterostome (“second mouth”) 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 (“first mouth”) development, and was the original form of embryonic development, as seen in all animal phyla thus far in Evolution 101. Here, the fate of all embryonic cells are sealed early on.
Besides their surprising deuterostome development, echinoderms boast unique features, such as an internal water canal system branching into tube feet which together enable movement and feeding. 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). Super weird.
As I teased just moments ago, we now move into the evolutionary territory of chordates which includes everything from dinosaurs to dingos. These are animals with a nerve chord (which develops into the brain and spinal chord), a flexible notochord (the start of a backbone), pharyngeal slits (which function as gills or inner ears depending on your species) and 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 lancelets: little blade-like critters which burrow backwards into the sand, leaving just their mouthparts exposed to catch passing food particles.
Along next were the sea squirts, or Urochodata. You may also know them as tunicates. 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 “eats” its own brain and major anatomical features – including the tail, notochord and a primitive eye – are absorbed back into the body. It spends the rest of its life immobile, siphoning water through its body and extracting food particles.
The next major development in animal evolution is represented by Myxini, or the hagfish: a jawless deuter with a skull made of cartilage. Take a look at this beauty queen.
The hagfish is a slime-producing marine animal that, while incredibly ancient, still exists as a number of different species today. It has rudimentary vertebrae, a brain, eyes and other sensory organs. In evolutionary terms, that’s kind of a big deal.
Our next guest is also a face for radio. Lampreys, or Petromyzontida, are other early adopters of backbones, and let’s not forget, 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.
Next came the evolution of jaws and a mineralised skeleton. Welcome to the age of true predators. Early Chondrichthyes (namely sharks and rays) were able to eat big chunks of flesh, so grew bigger and swam faster compared to their ancestors.
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.
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, Marjorie Courtenay-Latimer, who said: “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.”
By now, the most interesting 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 lobe-fins and land-dwelling tetrapod animals.
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 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.
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.
If gooey eggs tied early amphibians to the water, then hard-shelled eggs would liberate their descendants: the massively diverse phylum of Reptilia. I’m 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. They 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, so it lived alongside dinosaurs, and its body plan remains virtually unchanged today. In evolutionary terms, that means it has not accidentally discovered any better adaptations for life in the ocean than it already had 120 million years ago.
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 split dinosaurs into two major groups. First up, Ornithischia (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.
The other group of dinosaurs were the Saurischia (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.
For a long time, Aves, or birds, were thought to have descended from lizard-hipped dinosaurs, although recent research suggests we need to shuffle our evolutionary conception. Ironically, they may have descended from certain bird-hipped dinosaurs after all.
What we do know for sure is that birds evolved some remarkable bodily adaptations to aid flight, which has massive survival benefits: hunting, escaping predators, and widening their territories, to name a few. Hollow bones, lack of teeth, a single ovary in females, and specialised feathers all help to get birds off the ground, where so many dangers lurk.
Take a look at native New Zealand birds, for example, who evolved in geographical isolation for 80 million years and didn’t have to worry about any predators. Many species, such as the kiwi and kakapo, are flightless. So when 19th-century Europeans arrived with cats, rats and dogs, flightless bird populations crashed. It’s a demonstration of flight as an incredible survival adaptation.
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 they 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 large dinosaurs went extinct due to major environmental shifts. The mammals that survived exploited the ex-dinosaur habitats, food sources and territories, rapidly filling the ecological gaps left behind. This was our chance to shine.
This brings us to the present day. Congratulations for getting this far, both in terms of evolutionary history and reading this Evolution 101. You, sir, are fit to survive.
As you can see: from choanoflagellates to chameleons, life finds a way. Yes, I’m quoting Jurassic Park, the best movie of all time. 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.
In fact, the majority of life that has ever existed 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, including humans. Until now.
Are We Still Evolving?
David Attenborough suggests that, for the first time in billions of years, we may have stopped evolving. Of course, we still make mistakes in DNA replication and this creates novel mutations. But our species now wields considerable control over our environment so that the effect of natural selection is barely felt in developed nations. Agriculture ensures a steady food supply. Vaccines ensure herd immunity against infectious disease. Criminal justice ensures the weak aren’t preyed upon by the strong. In future, CRISPR 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.
Mother Nature can no longer kick us out of the gene pool when she feels like it. Instead, we swim to our hearts’ content, few of us pausing to realise that billions have lived and died before at the mercy of evolution.