Several Short Sentences About… Evolution

Over the years I’ve written several articles summarizing some of the most amazing things I’ve learned about the natural world, using the “several short sentences” format (though I confess my sentences, including this one, are generally anything but short):

• Several Short Sentences About Jellyfish
• Several Short Sentences About Bats
• Several Short Sentences About (Greenland) Sharks
• Several Short Sentences About Seeds

Since I’m constantly amazed about the strange things that have evolved over the 4.5B years since life first appeared on the planet, I thought I’d do one about evolution.

1. Life started in the ocean, but there are still some creatures coming ashore, and some other creatures migrating back to the water. Until about 500my ago, there was nothing much on land to eat except algae, bacteria, and mushrooms. So perhaps it’s no surprise that sharks have been around much longer than trees. When climate change finally enabled the growth of land plants (about 400my ago), near-shore aquatic animals evolved to be able to walk on land to be able to eat them (and to be able to escape marine predators). They mostly kept their gills but also evolved lungs to breathe in both environments (amphibians still have both). But in some cases animals that evolved on land have now evolved to live back in the water instead — notably whales and dolphins. An early mammal called Pakicetus (50my ago; image below left) evolved over the next 15my into a mammal called Dorudon (ancestor of modern whales; image below right) as it slowly migrated back to the ocean (images from UK Natural History Museum). Today’s hippos — much closer in their DNA to whales than to ruminant mammals — appear to be slowly migrating back to the water as their eyes and nostrils migrate up and back in their bodies.

2. The capacity of some animals to generate and detect electric fields evolved about 500my ago. It independently evolved eight times in evolutionary history (that we know of), and works differently in each case. Many marine animals like sharks can sense electric fields in order to detect the presence of objects, including predators and prey. Some electro-receptive fish can see these fields so clearly that they can swim backwards, ‘seeing’ as well with these fields as with their eyes. Others send out electric pulses (“active electroreception”) to detect objects in the electric fields they create. Still others, like the electric eel (not actually an eel) and some rays, can generate electric pulses strong enough to stun or kill predators and prey. And some can generate electric fields defensively, just to scare off predators into believing they could be harmful when they aren’t. Scientists are discovering that many of these creatures also use these electric fields as a communication device, modulating the waveforms of their electric pulses to send different messages. While mostly appearing in fish, this capacity survives in animals like dolphins and platypuses. And bees have recently been discovered to be able to detect electrostatic charges in flowers. And of course, there’s birds’ navigation abilities, which entail orientation to the Earth’s magnetic field, possibly at the quantum level, within the birds’ bodies.
3. While the evolution of photosynthesis in leaf-bearing plants is relatively recent (400my ago), photosynthesis evolved in some of the earliest-known forms of life on the planet, including algae and some bacteria. An even earlier type of photosynthesis, one that did not produce oxygen, is estimated to have started at least 3500my ago and lasted for 1000my. It’s hypothesized that during this low-oxygen period in the planet’s evolution, the atmosphere and coasts of the planet, seen from space, would have appeared purple rather than blue and green.
4. What followed our “purple period” is what is called the Great Oxygenation Event, or Oxygen Catastrophe. About 2500my ago, cyanobacteria evolved the capacity to produce energy from light in a new way, called oxygenic photosynthesis, that released oxygen into the air as a byproduct. The problem was, oxygen was toxic to most of the then-existing early forms of life, resulting in a massive extinction, and the atmospheric changes also led to a global glaciation (no, not Snowball Earth — that came much later). Some hypothesize that the only reason life wasn’t completely eradicated during this period was that the anaerobic archaea (archaea are living organisms that predate both plants and animals) evolved a symbiotic arrangement with the aerobic proteobacteria, called symbiogenesis, that allowed the symbiotic creature, called a eukaryote, to survive the new atmosphere. A by-product of this symbiosis was mitochondria, the necessary precursor to all complex forms of life (plants and animals) that we know of on Earth. Were it not for the accident of the Oxygen Catastrophe, our planet would likely still only be inhabited by single-celled life forms. Or by who knows what?
5. Thanks to symbiogenesis, simple animals now began to evolve on the planet (about 2200mya). But not plants. Another accident was required for the emergence of plants, and it occurred about 1600mya, when the symbiotes that had been produced in response to the Oxygen Catastrophe evolved a yet-more-complicated symbiotic relationship with those same cyanobacteria that had created the Catastrophe, yielding a new kind of eukaryote, one containing chloroplasts, which is the precursor to leaf-bearing plants. So now the stage was set for the emergence of complex life on Earth. But the drama wasn’t over yet.
6. There is great controversy over the theories that, three times between 720mya and 540mya, the entire planet cooled to the point it was completely, or nearly completely, covered in ice (or at least ‘slush’), including the oceans. This was at a time when the first multi-cellular life was emerging, including algae, mushrooms and jellyfish. Detractors of the theory claim there is insufficient evidence, given what we now know about the continents’ shifts, that the freeze was global, or that if such a thing happened, it would be a ‘runaway’ event, and the planet would have remained in its frozen state ever since. But there are explanations that accommodate these objections, and explain how the cooling arose in the first place, mostly relying on the idea of supervolcano activity radically altering the atmosphere and the amount of solar radiation reaching the Earth. But some of the explanations posit that it was the early evolution of multi-cellular life that itself precipitated the extreme cooling. We can never know. All we know is that, when this era ended with sharp global warming, massive melting, and release of minerals into the nearly-lifeless world of the time, what followed was the greatest expansion of diverse complex life our world has seen: The Cambrian Explosion, just over 500my ago.
7. The most oft-cited example of an exaptation is that bird feathers initially evolved for temperature regulation, but later were adapted for flight. I have postulated that perhaps the most astonishing exaptation in humans was the entanglement of our human brains, which might have occurred to enable us to deal with new, unfamiliar, harsh environments, but which led to humans being able to imagine unreal things (right brain) and then conceive of those imagined things as being real (left brain). This enabled most of our modern technologies (such as abstract language, and music, both of which are exaptations in their own right) but also enabled us to conceive of our imagined selves as real and separate from everyone and everything else. There is evidence that we are the only living creatures that do this. Whether that’s a capacity or an incapacity is debatable, but it does explain how our behaviour has diverged from that of other creatures, notably our closest cousins the chimps and bonobos (our genetics diverged from theirs only about 7my ago, a blink in time). The millipedes that first made the migration from ocean to land had evolved a jointed exoskeleton that suited their particular marine environments, but this exoskeleton was also well-suited to adapting to life on land — another exaptation. Stephen Jay Gould suggested that there is an exaptation/adaptation “cycle” that helps evolution occur more quickly, and also speculated that what we think of as our “junk DNA” (parts of our genetic makeup with no obvious function) might in fact be “spare parts” that can readily be put to use in this cycle.
8. About 7-8my ago, the Earth was bombarded by massive amounts of cosmic radiation, most likely from an exploding supernova star (and there was a similar massive cosmic storm 2-3my ago due to another supernova explosion). This storm occurred just as we were genetically separating from bonobos and chimps (or perhaps our separation at that time is not just a coincidence). There is evidence that this cosmic radiation produced ubiquitous lightning that caused a huge number of wildfires — perhaps enough to turn the heavily-forested African continent of the time into the mostly savanna grasslands which we still see today. This might have led both to an evolutionary preference for bipedalism (to see over the tall grasses), and an expanded and more protein- and amino-acid rich diet (dependent more on fish and less on figs and nuts) suitable for evolving a larger and more complex (and entangled) brain.
9. Ice ages are not a new or rare phenomenon in Earth’s evolution. In addition to the ones described in points 4 and 6 above, we know of three others: about 430-460my ago, 260-360my ago, and, much more recently, from about 34my ago until today. Each has precipitated or been accompanied by great extinction events. The chart below (using a crypto-log scale for time) is from Glen Fergus on Wikimedia (CC BY-SA 3.0). You can see the extraordinary climate instability and record cold that prevailed for most of the Pleistocene Epoch, and the extraordinary climate stability (until 1900CE) for all of the Holocene (the last 10,000 years). Humans emerged at the start of the Pliocene (black area on the chart below) but we apparently struggled to deal with the increasing cold, and when the Earth reached its coldest (and driest) temperature about 900ky ago, our human population apparently dropped to not much more than 1,000 people, eliminating 2/3 of human genetic diversity and an estimated 98.7% of the human population. And then, just as human population had started growing again, the supervolcanic Toba Eruption led to a second bottleneck about 70ky ago, with human population again dropping to a few thousand.

10. The current guess is that somewhere between 1.5my and 2my ago, humans evolved to become largely hairless, unlike our furry cousins. A genetic switch converted most of our hair follicle cells into sweat gland cells. This enabled us, scientists hypothesize, to survive and hunt better in the savanna left behind when the tropical forests where we first lived burned (point 8 above) and left us needing to work much harder, and under the open sun, to feed ourselves to survive. As we became bipedal, our exposed heads kept the hair needed to protect us from UV and other solar rays. Our exposed skin darkened to protect the rest of our bodies. We still have all the DNA needed to produce humans with heavily-furred bodies, but, for now, those genes aren’t expressed, so we remain the “naked ape”.
11. One of the great mysteries of human evolution is why our brains grew rapidly (tripled in size) from about 2my ago until about 160ky ago, and why (some claim) they’ve been shrinking since then. There are theories about this based on changing human environmental, social, and cultural conditions and behaviours, but none of them is satisfactory. Every theory put forth fails to explain why other creatures with the same evolving conditions and behaviours did not undergo similar significant brain size changes. Increased brain size and complexity makes huge demands on our metabolism, so its occurrence is unlikely to be accidental. One study insists the ratio of brain size to overall body mass has actually never significantly changed in our species, and that “size doesn’t matter; it’s what you can do with it” when it comes to brains’ capacities. Some birds’ intelligence is a pretty good indicator of this.
12. One of the great challenges in tracing the evolution of many creatures, and of human ancestors and relatives like chimps in particular, is that their populations were never that large to begin with, and they lived mostly in tropical areas that are not suitable for fossil formation. They, and we, evolved to live in specific, small, ecological and climate niches, to be bit players in the vast panorama of life on Earth. Our cousins knew better than to leave. We knew better than to stay.
13. The strange life cycles of periodical cicadas arose independently at least eight times that we know of — those life cycles are 13 and 17 years, and it is thought that these prime numbers were evolutionarily selected for because it is harder for both predators and prey to adapt their own shorter cycles to capitalize on or avoid the devastating emergence of cicadas. The synchronized emergence of these creatures after such a long period underground living off tree roots, only to die off completely after a few weeks of breeding the next generation, is still largely a mystery.
14. We continue to find mind-boggling examples of evolutionary adaptation every year, many of them now in the ocean’s deepest trenches, where pressure is enormous and light almost non-existent. One example is loricifera, a recently-discovered tiny deep sea creature with a head, mouth, brain and digestive system that breaks the rules of symbiogenesis (point 4 above) — they are multicellular organisms that have no mitochondria, and require no oxygen to thrive, using a completely different and more ancient means of respiration. Another example is siphonophores, a diverse and complex grouping of sea creatures that vary from 2mm to 50m in length (longer than the largest whales). They are designated an order, with 175 known species, but they might better be described as a community of interdependent, separately specialized creatures. But then, we might describe the human animal the same way.
15. And if all that is not mysterious enough, there are some species that have evolved the ability to procreate through asexual parthenogenesis — no males required. And it’s not rare: “Parthenogenesis occurs naturally in some plants, algae, invertebrate animal species (including nematodes, some tardigrades, water fleas, some scorpions, aphids, some mites, some bees, some Phasmatodea, and parasitic wasps), and a few vertebrates (such as some fish, amphibians, reptiles, and birds”.
16. Most mammals, birds, insects, reptiles and amphibians need sleep. Fish apparently can and do sleep, but don’t suffer any adverse effects when they don’t. But bullfrogs apparently don’t sleep at all. And whales and dolphins can go a month without sleeping. Ducks often sleep half a brain at a time, with one eye and one brain hemisphere awake and alert. So, apparently, do crocodiles and some sharks. Bats sleep an average of 19 hours a day; giraffes, only 3 hours a day. No one seems to know if, or how, migratory birds sleep while they’re flying long distances without breaks. And while we know why we seem to need sleep, no one seems to know why we haven’t evolved species that don’t need sleep, which presumably would be an evolutionary advantage.
17. When their local environment and local climate don’t change, some creatures appear to hardly evolve at all, even over millions of years (some crabs, turtles and fish for example). At the other extreme, one reason that bacteria have been the most successful animal inhabitants of our planet for its entire history (3450my) is their staggering capacity to evolve quickly, including the capacity to transfer genes from one bacterium to another. Another reason is their capacity to adapt to many different niches of climate and environment, including some extremely hostile ones. The current great extinction is likely (barring runaway climate change that creates a Hothouse Earth) to create huge niches for new life forms to evolve to fill, as past extinctions did. Scientists believe rodents, cockroaches, termites, bats, and pigeons will continue to thrive in a hot, tempestuous, future Earth (largely thanks to us, their populations have recently exploded and they’ve developed mutations to adapt to many different environments). The large cats might return and replace us as apex predators. Rodents and other surviving land animals might evolve to follow the hippos back to the oceans if life on land becomes too hot, dry and stormy. And animals might evolve with the capacity to extract the vast stores of carbon in all the waste plastics we’ve created that will likely be around long after we’re gone.

The word evolution means unrolling. Charles Darwin didn’t use the term, and disliked it because it implied to him the idea of ‘progress’, and his theory was about change and adaptation — he was indifferent to the idea that it had any ‘direction’, and actually used the expression “descent with modification” instead. No wonder his work was so loathed by the churches of the day!

In our use of the term, we have re-embraced the idea of progress, and adopted evolution “trees” which, absurdly, show humans at the “top” of the tree, as the “crown of creation”. The idea that it’s a random process, without direction or purpose, with cycles of increasing and decreasing complexity, is too challenging for many humans to accept.

So perhaps unrolling isn’t a bad definition of evolution. It’s just that it’s a form of unrolling, irrespective of whether or not there are any humans around to witness it, that, at least at a cosmic level, has no beginning, no direction, and no end.