Evolution Explained: From Bacteria to Humans

Evolution Explained: From Bacteria to Humans

Evolution connects all life on Earth. Whether you're a marine worm or a marmoset, the same genetic code proliferates your DNA.

The Primordial Soup

Once upon a time, at least 3.5 billion years ago, there was a primordial soup. This watery mess contained all your favourite ingredients.

Illustration of what was in the primordial soup: carbon, hydrogen, oxygen, water, carbon dioxide, ammonia, and methane

The essential molecules of life were all present in the primordial soup.

The soup was heated, perhaps by lightning, fire, lava, or hydrothermal vents. The heat initiated chemical pathways that drove the spontaneous formation of genetic molecules called nucleotides.

You may know them as Adenine (A), Uracil (U), Cytosine (C), and Guanine (G). They accumulated in abundance, layer upon layer, as brownish crystals on the surfaces of rocks.

Eventually the nucleotides bonded with phosphorus in the rocks to create the first RNA strand.

Illustration of the first RNA strand

The first RNA strand was a chemical inevitability.

In time, the churning soup produced more variations of RNA with varying shape, length, and sequence. Over millions of years, an array of RNA machines emerged. This is theory is known as RNA World.

The First Cell

Meanwhile, long chains of carbon, hydrogen, and oxygen atoms formed to make fatty acids. They bonded with phosphate groups to make this little guy, who would be essential in making the first cell.

Illustration of a phospholipid with a hydrophilic phosphate head and a hydrophobic fatty acid tail

Phospholipids self-assemble all the time in nature to create membranes.

You've seen it happen in your bubble bath: lipid molecules form spheres when they come into contact with water. The same hydrophobic interactions caused phospholipids to form tiny bubbles in the primordial soup, giving rise to the first cell membranes.

And when molecules of RNA became trapped inside these membranes, the first cells were born.

Illustration of the first cell on Earth

The first cells were simple membranes containing basic RNA.

They were simple cells, but cells nonetheless. Bacteria, in fact, complete with strands of RNA.

This is the theory of abiogenesis: the origin of life from non-living molecules.

And the race to evolve began.

The Three Domains of Life

Over hundreds of millions of years, mistakes made by the self-replicating RNA gave rise to novel cell features. Some of these cells were so diverse from the first bacteria, they landed themselves a new name altogether: archaea.

This gave Earth two domains of life: bacterial cells (named for the Greek word bakterion which means "small rod") and archaeal cells (named for archaeo in Greek which means "ancient").

It was a drizzly Tuesday afternoon when an archaeal cell engulfed a bacterial cell. She just clean swallowed him whole inside her membrane, and in doing so produced a third distinct domain of life: eukarya. You and I are eukaryotes, as are all plants, fungi, and animals.

Illustration of Endosymbiotic Theory, where an archaea engulfed a bacteria to produce the first eukaryote

The merging of an archaeum and a bacterium produced an entirely new type of organism.

It was a total accident. Yet both cells found their newfound endosymbiosis rather agreeable. The archaeum derived energy from her new tenant, while the bacterium was protected inside his new house.

It was an entirely new mode of survival, giving the resulting eukaryotic cell a more complex internal structure and a second powerhouse of energy.

Today, bacteria, archaea, and eukarya make up the three domains of life on Earth.

Phylogenetic tree illustrating the three domains of life: bacteria, archaea, and eukarya

This phylogenetic tree shows the relationships between the three domains of life.

How Does Evolution Work?

By now you have an idea of how spontaneous chemical interactions drove the origin of life. So where does evolution come into it?

Let's briefly skip forward in time to visualise evolution in terms of organisms much more familiar than single-celled organisms. Let's talk animals.

Today we arrange animals into different species based on their genetic differences. So how do these differences arise if all animals ultimately share a common ancestor?

The answer is evolution: the gradual change in how species look and behave over time, initiated by random genetic mutation and refined by natural selection in the environment.

How Does Evolution Work? An illustration of how mutation (new genes) creates adaptations (new traits) which fuels evolution (new species)

Evolution is the grand cumulative result of new genes and new adaptations over time.

It's a wild thought. At the most basic level, evolution is fuelled by chaos: by mistakes in genetic replication.

Natural selection brings order to that chaos. The base theory of evolution (that organisms change over time) was around for a century before Darwin and Wallace came along. These two naturalists concluded independently that nature prunes away detrimental traits and rewards beneficial adaptations with greater survival.

This survival of the fittest explains the paradox of how increasingly complex life arises from random genetic mutation.

Darwin and Wallace cartoon

Darwin and Wallace had a strange relationship.

Let's dive a little deeper into this idea. Evolution is fundamentally a two-step process:

1. Blind Mutation

Mistakes in DNA replication don't always lead to evolution. In fact, they often cause disease. Even the erroneous insertion or deletion of a single nucleotide can muddle the sequence that follows, throwing entire (genes) out of whack. Examples of single-gene mutations in humans include cystic fibrosis and colour blindness.

But not all mutations are bad. Just look at the X-Men. Sometimes, switching out a nucleotide makes no difference at all. These are called silent mutations.

And sometimes, mutations translate to the production of new proteins, creating adaptations like scales or teeth. Other mutations alter the body plan, creating adaptations like an extra toe or faster reaction time.

But mutation is always blind, like infinite monkeys smashing at infinite typewriters. Eventually, a monkey will type a readable word. It wasn't deliberate, and yet it produces a meaningful combination of letters in the genetic sequence.
Cartoon of infinite monkeys on typewriters to demonstrate the genetic mutation is always random

Random mutation can be compared to the literary works of haphazard simians, whose combined word count provides the raw material for natural selection.

2. Natural Selection

Now nature can prune away the life forms with disease mutations, who can't flourish in the environment. They're less likely to reproduce, and their genes are sooner flushed out of the gene pool.

Conversely, useful adaptations make organisms stronger, healthier, or faster, thereby raising their chances of survival and reproduction.

A well-camouflaged iguana has better odds of evading the eye of a predator, making him more likely to reproduce and pass on his mutant genes. Nature has selected him for success.

Together, blind mutation creates the diversity of life, and natural selection provides the self-correcting mechanism for adaptive progress. When sufficient adaptations accrue in a population, they become a new species.

While it may seem like genes are goal-oriented (or selfish in the words of Richard Dawkins), they're really just chemicals in a feedback loop. There's no grand plan in evolution; only billions of life forms being shaped by their changing environment.

The Evolution of Animals

Back to our story. How did the single-celled eukaryotes evolve into the variety of plants, fungi, and animals on Earth today?

I'm glad you asked. While some eukaryotic cells thrived in the singular form, others adapted to live in colonies, producing the first multicellular organisms. This was kind of a big deal:

  • Plants were born as aquatic algae before evolving on land as mosses. The evolution of roots took them to new heights: first as vascular plants (ferns), then seed plants (conifers). Angiosperms (flowering plants) came 250 million years ago—some dinosaurs never saw flowers.

  • Fungi likely began as tiny aquatic cells with tails, allowing them to move towards food and away from hostile environments. They, too, went colonial and took to the land, evolving into complex yeasts, moulds, and mushrooms which play critical roles in our ecosystem today.

  • Animals emerged about 635 million years ago, when single-celled eukaryotes called choanoflagellates had a smashing idea...

Illustration of a choanoflagellate, the single-celled life form that gave rise to the first animals on Earth

Choanoflagellates were single-celled organisms that gave rise to the first animals on Earth.

Choanoflagellates began living in colonies, finding greater safety in numbers. These clumps of cells eventually evolved specialised functions, each sub-cluster providing different benefits to the colony as a whole.

We now identify these colonies as the most basic group of animals: Porifera, or sponges to you and me.

You may not think of sponges as animals. They have no brains, blood, organs, or even true tissues. Yet they mark the origin of another new way of life: clustered cells with differentiation, giving rise to new emergent properties. By Jove, life found a way.

Roald Dahl had it right when he named Aunt Sponge. This is your most distant animal relative in the world.

Porifera Evolution Explained: the features of sponges are specialised cells, a protein matrix, internal canals, and plasticity

Porifera are 635 million years old.

The Evolution of Radial Symmetry

As sponges mutated and gained complexity and size, some of them took on new qualities like softer bodies and radial symmetry. This symmetry gave them a distinct top and bottom.

A new phylum of animals emerged, called Cnidaria. They include all jellyfish, hydras, anemones, and corals around today.

Cnidaria Evolution Explained: the feature of cnidarians are radial symmetry, two body layers, stinging cells, and distinct lifecycles

Cnidaria are 580 million years old.

Cnidarians also evolved specialised stinging cells, called cnidocytes, which help them capture food actively. Stingers marked the dawn of predatory behaviour in animals, a factor that would drive many later species into an evolutionary arms race.

Also for the first time on Earth, animals boasted distinct tissue layers, giving them an internal body cavity for digesting food and transporting nutrients. Now that's some fancy biological equipment.

The Evolution of Bilateral Symmetry

Over millions of years, Cnidarians diversified further into exotic forms. It's thought that hydras evolved into flatworms, which rule their own entire phylum called Platyhelminthes. The major new evolutionary feature of flatworms was bilateral symmetry.

Platyhelminthes Evolution Explained: features of flatworms are bilateral symmetry, cephalisation, a primitive brain, and a central nervous system

Platyhelminthes are at least 550 million years old.

Bilateral symmetry gives an animal mirrored left and right sides. You and I have bilateral symmetry. It's a massively useful adaption that led to the development of a head-end, also known as cephalisation.

Until this point, radial symmetry suited a lifestyle of drifting through the ocean, meeting the environment equally from all sides. But bilateral symmetry and cephalisation lent animals to active movement. They encountered their environment head-on, where all the sensory equipment was located.

With eyes and mouths at the head-end of the flatworm, nerve bundles also became more usefully concentrated in the head too. These clusters of nerve cells produced an early—if primitive—brain.

The Evolution of Parasites

Parasites go way back. It's hypothesised that Nematodes evolved from animals whose development was arrested in the larval stage. However, their true age and precise lineage is unknown because their fossilised remains are just so damn small.

Nematodes are tiny critters, known for their cylindrical bodies and adaptability to a diverse range of habitats. Different species are content living deep in the Earth's crust, in the ocean, or in your gut.

Nematode Evolution Explained: the features of roundworms are a tough outer cuticle, complex eyes, a partial body cavity, and a complete digestive system

Nematodes are at least 400 million years old.

One evolutionary superpower of Nematodes is a tough outer coating called a cuticle which protects their inner organs.

They also hit on the parasitic lifestyle, stealing energy directly from their hosts. Roundworms are simple yet highly adaptive creatures, and that's precisely what makes them so successful.

The Cambrian Explosion

Half a billion years ago, there was a biological Big Bang. The evolutionary tree of life spawned myriad new branches, including dozens of new animal phyla. All the major body plans we see in animals today emerged during this incredible period of evolution. So what kicked it all off?

The Cambrian Period started around 540 million years ago and lasted for about 55 million years. The idea of it being a Big Bang or an explosion is somewhat of a relative metaphor, but in terms of evolution, a lot happened over a very short time.

Graph showing the increase in the number of animal phyla during the Cambrian Explosion

The fossil record shows an explosion of animal diversity during the Cambrian era.

At the start of the Cambrian era, Earth was cold. But soon, global temperatures climbed, glaciers melted, and sea levels rose.

The shrinking ice sheets allowed more light to penetrate the oceans, having major consequences on aquatic life:

  • More sunlight fuelled the growth of more aquatic plants
  • More plants provided more grazing food
  • More plants created more oxygen via photosynthesis
  • More oxygen allowed animals to grow larger

The Cambrian Period was a perfect storm of abiotic (non-living) factors like climate and topography, interacting with biotic (living) factors like the food web and predation. Such feedback cycles drove the rapid diversification of the animal kingdom.

As you'll see in the next few phyla, the rapid co-evolution of animals pushed predators and prey into an arms race. Predators grew bigger, stronger, and faster, while prey developed camouflage, protective shells, and faster reflexes.

Genetic Evolution During The Cambrian Explosion

At the genetic level, things were changing fast. In particular, there were some key mutations in homeobox genes. These control the development of the body plan during the embryonic phase. In other words, they organise the head from the arse.

Because homeobox genes are the master switches of physiology, just small mutations in them can create massive differences to the overall body structure. Homeobox mutations eventually gave bodily segmentation to worms, and an extra set of wings to insects.

Homeobox changes may have been in the making before the Cambrian Period began. When the Earth's environments and ecosystems changed, homeobox variations provided superb fodder on which natural selection could act.

The Evolution of Shells

Sometimes when you walk along the beach you find a really nice shell. These are not geological fancies but rather the calcium carbonate shells of dead Molluscs. That's right—beaches are mass snail graveyards.

The evolutionary features of Molluscs include soft bodies which excrete a hard protective shell. Most species are aquatic (like clams, oysters, and squid) although some live exclusively on land (like your standard garden snail).

Mollusc Evolution Explained: features of Molluscs are a calcareous shell, a visceral mass, a radula, and a mantle

Molluscs are 550 million years old.

Wait a minute, Columbo, did you just say squid?

Yep, squid are Molluscs too. The squid has a small ancestral shell, called a gladius, which became internalised in the course of its evolution. The gladius supports the mantle and serves as a point for muscle attachment.

Squid are incredible creatures that independently evolved eyes. And they can grow huge. In the case of colossal squid, up to 14 metres in length. Squid are closely related to octopuses, who not only have three hearts and blue blood, but have also demonstrated problem solving and tool use. Molluscs can be extraordinarily intelligent creatures.

The Evolution of Body Segments

The early Cambrian Period gave rise to a lot of worms. Annelids, including marine worms and deliciously slimy earthworms, evolved distinct body segments, each one internally and externally identical to the next.

Annelid Evolution Explained: features of Annelids are a segmented body, three tissue layers, circular muscles, and a circulatory system

Annelids are 518 million years old.

Some marine annelids evolved tiny paddle-like feet for swimming, which would later enable them to push through soil on the land. In time, the evolution of Annelids would create an enormous range of body sizes, from 0.5-millimetre aquatic worms to 3-metre Giant Australian Earthworms.

The Evolution of Appendages

The paddle-like feet of marine worms kicked off an incredible phylum, members of which reside in your home today: Arthropods.

The three main lineages of arthropods are spiders, insects, and crustaceans. Together they account for 80% of all animals on Earth today. In other words, Arthropods rule.

Arthropods first appeared when some Annelids evolved into tiny crustaceans called ostracods, or seed shrimp. Homeobox mutations saw their paddles develop into jointed appendages which later specialised to function as antennae, pincers, mouth-parts, and legs.

Arthropod Evolution Explained: features of Arthropods are an exoskeleton, jointed appendages, gills and book lungs, and compound eyes

Arthropods are at least 450 million years old.

Some Arthropods prospered in the water: shrimp, trilobites, and sea spiders still flourish there today. Crabs adapted to spend part of their days on land. Spiders, scorpions, and beetles went terrestrial full-time. And land-dwelling species gave rise to flying insects like bees, butterflies, and mosquitoes.

Arthropods make up a huge and diverse phylum. Yet they all share key features which allow us to trace their common descent. They're exceptional at mastering life on Earth, being the only phylum to truly conquer land, sea, and air.

The Evolution of Embryonic Development

All the animal phyla we've looked at so far go through an embryonic stage known as protostome development. It characterises the way embryonic cells are organised to create a basic body plan with various internal layers.

If we took a spider embryo when it's just eight-cells small, we'd see the cells arranging themselves in a spiral shape. These cells multiply many times to form a hollow sphere, and then fold inwards to create the beginning of the animal's mouth.

Illustration of protostome development in animals: spiral cleavage occurs at the 8-cell stage and mouth develops first in the gastrula stage

Two hallmarks of protostome development.

The Cambrian boomers changed all that, ushering in a new era of embryonic growth called deuterostome development 500-600 million years ago. You and I are deuterostomes. Our eight-cell stage is defined by a radial pattern, and later, the first infolding is set to become an anus.

Illustration of deuterostome development in animals: radial cleavage occurs at the 8-cell stage and anus develops first in the gastrula stage

Two hallmarks of deuterostome development.

Why does this matter? Because it changes the way our internal layers form. Deuterostome development affords us more a complex nervous system and, ultimately, a backbone. This is what separates most vertebrates from invertebrates. And it's all thanks to mutations in the homeobox genes.

Echinoderms—also known as starfish and sea urchins—undergo deuterostome development.

Echinoderm Evolution Explained: features of Echinoderms are deuterostome development, spiny skin, tube feet, internal water canals, and a central nerve disc

Echinoderms are 540 million years old.

You read that right. Sea stars are pretty advanced animals in the grand scheme of things. They're not vertebrates, but they do mark the evolution of key transitional states on the way towards it.

Besides their evolution as deuterostomes, Echinoderms boast an internal water canal system and tube feet which together enable movement and feeding. They also reproduce sexually by releasing sperm into the ocean, as well as asexually by breaking off limbs and regenerating. Amazing.

The Evolution of Chordates

With a more complex mode of embryonic development at hand, a new phylum called Chordates evolved. It includes a few more bizarre animals, plus many familiar ones, including dinosaurs, ducks, and dingoes.

Chordates possess four key features:

  • The hollow nerve chord develops into the brain and spinal chord.
  • The flexible notochord develops into the backbone.
  • Pharyngeal slits function as gills or an inner ear.
  • The post-anal tail aids swimming and balance.

At a glance, early chordates don't look that special. Take a look at the sea squirt or Urochordate. The larval form has all the Chordate features as a tadpole-like creature in search of a landing pad. This immature stage may only last a few minutes. Then something freakish happens.

The sea squirt undergoes a radical metamorphosis, re-absorbing its tail, notochord, and primitive brain back into its body. The rest of its days are spent as an immobile little squirt, siphoning water to filter food, and looking like a gross internal organ while it does so.

Urochordate Evolution Explained: features of Urochordates are a cellulose tunic, oral and excretory siphons, larval Chordate features, and retrogressive metamorphosis

Urochordates are 540 million years old.

Another early Chordate example is the Lancelet, or Cephalochordate. This is a blade-like critter that burrows backwards into the sand, leaving its mouthparts exposed to catch drifting food particles.

Cephalochordate Evolution Explained: features of lancelets are pharyngeal slits, a notochord, a dorsal hollow nerve chord, and a post-anal tail

Cephalochordates are 530 million years old.

Lancelets have no true vertebrae, their brains are small and primitive, and they have pretty dull senses. Yet they do have all the features of Chordates like you and me, reminding us that evolution is a gradual process of cumulative adaptations.

Having glimpsed some of the more alien-like animals throughout our evolution, we now move on to the more familiar. But before we delve into vertebrates, just look how far we've come:

Phylogenetic tree showing invertebrate evolution

This phylogenetic tree shows the relationships between the major invertebrate phyla.

The Evolution of Vertebrates

Take a look at this beauty queen.

Myxini Evolution Explained: features of hagfish include a partial skull, a cartilage skeleton, sensory barbels, and defensive slime

Myxini are 500 million years old.

The hagfish, or Myxini to zoologists, is basically a swimming sausage. He's a jawless fellow with a partial skull made of cartilage and the ability to produce defensive slime.

What makes him special is the presence of rudimentary pseudo-vertebrae. Unusually for a sausage, he also has a brain, eyes, and other sensory organs. He's elusive in the fossil record, yet alive (extant) today, providing physiological evidence for his evolutionary origins.

Our next guest is also a face for radio.

Hyperoartia Evolution Explained: features of lampreys are a complete skull, a sucker mouth, keratinised teeth, and dorsal fins

Hyperoartia are around 500 million years old

Lampreys, or Hyperoartia, are early adopters of true backbones. The fossil record reveals their shape has stayed virtually unchanged over hundreds of millions of years of evolution.

Humans descended from a sister animal of lampreys, who was the first to develop an adaptive immune system, allowing our bodies to recognise and remember pathogens today. Although now extinct, they likely looked very similar to lampreys, meaning this is what your great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-grandmother looked like. You really have her eyes / tail / sucker mouth.

The Evolution of Jaws

About 430 million years so, natural selection favoured 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 raced along (relatively speaking, of course).

Chondrichthyes Evolution Explained: features of Chondrichthyes are placoid scales, flexible jaws, specialised fins, and a two-chambered heart

Chondrichthyes are 450 million years old.

The Evolution of Lungs

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

Sarcopterygii are muscular, lobe-finned fish with bony skeletons, such as lungfish and coelacanths. The latter was thought to have been extinct for 65 million years until, in 1938, a very confused fisherman hauled one in and took it to a local naturalist for identification.

Sarcopterygii Evolution Explained: features of Sarcopterygii are a bony skeleton, muscular fins, proto-lungs, and enamel teeth

Sarcopterygii are 418 million years old.

All other fish fall into the phylum of Actinopterygii, or ray-finned fish, which evolved manoeuvrable 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. However, for some, it performs as a rudimentary lung.

Actinopterygii Evolution Explained: features of Actinopterygii are horned fins, an ossified skeleton, a swim bladder, and specialised scales

Actinopterygii are 400 million years old.

The Evolution of Tetrapods

By now, the most complex life forms on Earth had eyes, brains, backbones, jaws, gills, lungs, and fins. Only relatively small modifications of this body plan were needed to produce the superclass known as Tetrapods (meaning "four feet").

The recent fossil discovery of Tiktaalik provided the so-called missing link between aquatic lobe-fins and land-dwelling Tetrapods. Like a fish, the extinct Tiktaalik had fins, gills, and lungs, and its body was covered in scales. But unlike a fish, it had rudimentary ribs to ventilate its lungs, seriously muscular fins to support its body out of the water, and a neck and shoulders to help move its head.

Notably, Tiktaalik's fin-feet already had the bone structure common to Tetrapod wrists today. The fellow gave rise to the first Tetrapod land dwellers known as Amphibians.

Amphibian Evolution Explained: features of Amphibians are jointed limbs, ventilated lungs, a three-chambered heart, and vocalisation

Amphibians are 368 million years old.

Amphibians include frogs, salamanders, and caecilians. While reasonably 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.

Amphibians also go through quite the metamorphosis early in life, having aquatic larval forms which are distinctly different from their adult forms. Such convoluted lifecycles are evolutionary throwbacks to older animals like jellyfish and tunicates.

The Evolution of Amniotic Eggs

If moist eggs tied Amphibians to water, then hard-shelled eggs would liberate their Reptiles descendants.

Reptile Evolution Explained: features of Reptiles are keratin scales, a complete rib cage, advanced colour vision, and hard shelled eggs

Reptiles are 320 million years old.

Reptiles include some of the most objectively fun animals that ever existed: dinosaurs, snakes, lizards, crocodiles, and turtles.

Consider Testudines for a moment: the bizarre shelled-reptiles known as turtles, tortoises, and terrapins. Once possessing teeth, Testudines lived alongside the dinosaurs and survived the mass extinction event that killed its Reptile cousins.

Testudines Evolution Explained: features of Testudines are a bondy shell, scutes, musk glands, and a retractable neck

Testudines are 200-279 million years old.

Dinosaurs are a completely different evolutionary branch of Reptiles that lived during the Mesozoic Era. Dinosaurs are split into two major groups:

  • Ornithischians (meaning "bird-hipped") were mostly herbivores. Some Ornithischians, like Triceratops and Ankylosaurs, evolved thick skulls and armoured plates which protected them from predation.
  • Saurischians (meaning "lizard-hipped") included carnivores like Tyrannosaurus Rex and Giganotosaurus, as well as massive long-necked herbivores like Diplodocus and Brachiosaurus.

The Evolution of Flight in Vertebrates

A number of dinosaurs are known to have evolved wings. For instance, Pterosaurs evolved flight some 230 million years ago, while Microraptors had feathered wings for gliding about 120 million years ago.

So how and when did birds evolve from dinosaurs?

As Alan Grant taught us in Jurassic Park, Aves descended from dinosaurs. In fact, Archaeopteryx is widely considered the first bird, and as you very well know, Archaeopteryx was also a dinosaur. As a result, most palaeontologists now accept that birds are a specialised group of dinosaurs that arose 160 million years ago.

Modern birds evolved from as few as three dinosaur lineages about 95 million years ago during the end of the Cretaceous Period. They have some amazing adaptations to aid flight, including hollow bones, loss of teeth, a single ovary in females, and aerodynamic feathers.

Aves Evolution Explained: features of Aves are feathered wings, hollow bones, warm blood, and a four-chambered heart

Aves are 160 million years old.

Flight has massive survival benefits in terms of hunting, escaping predation, and migration. Yet some predator-free environments have caused birds to become flightless. When it comes to evolutionary adaptations, you have to use it or lose it.

Kiwis are one of many flightless bird species native to New Zealand, which evolved in geographical isolation for 80 million years. When the Polynesians arrived on the islands with rats in the 13th century, native bird populations took a big hit. The introduction of cats and dogs by European settlers in the 18th century only exacerbated the problem.

Extinctions happen for a number of reasons, but there's always one common feature: the environment changes too rapidly for organisms to adapt. Humans certainly aren't making it easier.

The Evolution of Mammary Glands

By definition, Mammals are animals which have hair and produce milk from mammary glands. I don't know my dad's excuse.

Pseudo-mammals evolved from Reptiles 178 million years ago. By the end of the Triassic period, Mammals were small, hairy creatures which fed on insects at night and still laid eggs. In time, they diversified to a degree, but competition and predation was fierce: dinosaurs already dominated many ecological niches.

Three lineages of Mammals—Monotremes (egg-layers), Marsupials (pouch-bearers), and Eutherians (placenta-bearers)—were established when most dinosaurs went extinct due to sudden 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.

Mammal Evolution Explained: features of Mammals are hair and fur, mammary glands, a complex brain, and specialised teeth

Mammals are 178 million years old.

Over the next 65 million years, Mammals diversified into a range of forms and lifestyles. Rodents took to living underground. Primates took to the trees. And Cetaceans said "sod it" and went back to the oceans to become dolphins and whales.

It wasn't until 4.4 million years ago when Hominins emerged in the form of Australopithecus. These unusually smart African apes were likely the direct ancestors of modern humans. They thrived for 3 million years before disappearing at the hands of global climate cooling, or competition with other species of Hominins, or both.

Illustrated timeline of human evolution

Homo sapiens are the only surviving species of Hominins

Homo sapiens emerged just 200,000 years ago, according to fossil remains found in Africa. Yet it hasn't been a smooth ride for us either.

Around 75,000 years ago, our numbers were catastrophically diminished to between 3,000 and 10,000 individuals at the hands of the Toba supervolcano eruption in Indonesia. We hit a genetic bottleneck, evidence of which exists in your DNA today. Climate change and deforestation caused by the Toba eruption may have forced the surviving humans to adopt new survival strategies. These behavioural adaptations may even have driven us to replace the Neanderthals and other archaic human species.

The rest, as they say, is history.

Congratulations for getting this far. You, sir, are fit to survive. Every single ancestor of yours has matured and reproduced without fail for 3.5 billion years straight.

They didn't know it, but they were all working towards you... And the guy at the petrol station... And your barber... And the old lady who stalks you on Facebook... Ok, it doesn't sound so special anymore. But if you can have an ego about it, it can all seem rather wonderful.

The fact that we exist means we've won the evolutionary lottery. All 7.8 billion of us. Even if we do all hark back to a slimy old hagfish.

Phylogenetic tree showing vertebrate evolution

One more phylogenetic tree for the road. Here are the relationships between the major vertebrate phyla.

Are Humans Still Evolving?

The zoologist David Attenborough suggests that, for the first time, humans may have stopped evolving.

Wait a second—what?

We still make mistakes in our DNA replication. And this still creates diseases and adaptations. And let's not forget that evolution takes a heck of a long time to ripple through 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 natural selection is, for now, significantly diminished.

For billions of humans, agriculture and shipping ensure a steady food supply. Vaccines provide herd immunity against infectious disease. Criminal justice prevents dangerous individuals from reoffending. And gene editing allows us to correct disease mutations, and even our entire germ lines.

As long as our society and technology hold strong, 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 the sheer multitude of organisms that have lived and died before us at the mercy of evolution.

Becky Casale Author Bio

Becky Casale is the founder, keyboard smasher, and drinks lady at Science Me. She's a BSc undergrad and the mum of two catastrophically awesome critters.