Evolutionary history suggests intelligent life is rare in the universe

378589main_cepb_1920_full.jpgAre we alone in the universe? It all comes down to whether intelligence is a probable outcome of natural selection, or a fluke. Is evolution repetitive and predictable? If evolution repeats itself, independently converging on similar outcomes, our evolution seems likely. But 4.5 billion years of evolutionary history shows that many key events- not just the evolution of intelligence, but events preceding it, like the evolution of animals, complex cells,  photosynthesis, life itself- were unique. The evolution of intelligence may, therefore, be unlikely. Like winning the lottery, only less likely.

           The universe is astonishingly vast. The Milky Way has over 100 billion stars, and there are around 100 billion galaxies in the visible universe. That’s just the tiny fraction of the universe we can actually see. Huge, possibly infinite gulfs of space exist beyond, far enough away that there hasn’t been time for the light from those stars to reach us. Even assuming Earthlike worlds are rare, the sheer number of planets- there as many planets as stars, maybe more- suggests that a lot of life is out there. So where is everyone? This is the Fermi Paradox: our universe is vast, and old, with ample time for intelligent life to evolve, but there’s no evidence of anyone else out there.

 We can’t study alien worlds to know if they’re likely to evolve intelligence. But what we do have is billions of years of Earth’s history, and we can study the fossils and living things produced by that history, to see where life repeats itself, or doesn’t.

 

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The Tasmanian Wolf (Thylacinus cynocephalus). Although it looks like a wolf, it actually had young that lived in a pouch- because it was related to kangaroos, not wolves. The similarities are an example of convergent evolution.

            Striking examples of convergence do exist in evolution. The Tasmanian wolf was a wolf-like creature that filled the predator niche in Australia, but it has a kangaroo-like pouch. It evolved from marsupial stock- as did marsupial moles, marsupial anteaters, and marsupial flying squirrels. In fact, Australia’s entire evolutionary history, with mammals taking over and diversifying after the dinosaur extinction, parallels other continents. Other striking cases of convergence include dolphins and extinct ichthyosaurs, which evolved similar streamlining to slice through the water, and the birds, bats, and pterosaurs, which each evolved wings and flight.

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The squid eye looks and works much like a human eye, but built differently- because it evolved its eyes independently from our own.

 

You also see convergence in individual organs. Eyes evolved not only in vertebrates, but in arthropods, octopi, scallops, worms, and jellyfish. Vertebrates, arthropods, octopi and bristle worms independently evolved jaws. Legs were likewise invented convergently in the vertebrates, arthropods, and mollusks. In fact, several kinds of fish- tetrapods, frogfish, skates, mudskippers- specialized their fins for walking.

            But here’s the catch. All these examples of convergence exist within just one group of organisms- the Bilateria. Bilaterians are complex animals, with left-right symmetry, a mouth, a gut, a nervous system. Once they evolve, we see different animals evolving similar solutions to similar problems. But the evolution of the bilaterian body plan that makes this possible was a unique innovation- complex animals evolved once in life’s history.

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The vertebral column is a one-off event.

 

The argument that evolution tends to repeat itself makes sense- if you only focus on the places where it does repeat itself, like marsupial wolves, eyes, jaws, and soforth. But it’s a case of confirmation bias. If we only study cases of convergence, we’ll arrive at the idea that convergence is everywhere, and evolutionary history is likely to repeat itself. But if you start looking for nonconvergences- places where evolution hasn’t repeated itself, you’ll find them.

Surprisingly, many of the most critical events in evolutionary history are unique. One is the vertebral column that lets large animals exist on land. But only vertebrates evolved this particular kind of skeleton (curiously, the jointed exoskeleton, which underlies the most diverse group of animals, the insect, is also unique). The complex cells all animals and plants are built from- eukaryotic cells, with nuclei and mitochondria- evolved only once. Sex evolved once. Photosynthesis, which increased the energy available to life, and filled the atmosphere with the oxygen needed by complex animals, is an evolutionary one-off. For that matter, so is human-level intelligence. There are marsupial wolves and moles,  but not marsupial humans.

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The eukaryotic cell of amoebas is far more complex than that of bacteria- and humans and other complex, multicellular organisms are made out of eukaryotic cells.

            Furthermore, these unlikely events depend on one another. Humans couldn’t evolve without vertebrae to let fish move onto land. Skeletons couldn’t evolve before complex animals existed. Complex animals required complex cells. Complex cells didn’t exist until photosynthesis filled the world with oxygen. And nothing could happen without the evolution of life: a singular event among singular events. All living things come from a single ancestor: as far as we can tell, life only happened once.

Strikingly, these events seem to take an extraordinarily long time. Photosynthesis didn’t evolve until around 1.5 billion years after the Earth formed, complex cells after 2.5 billion years, animals after 3.9 billion years, and human intelligence, 4.5 billion years later. That these innovations are so clearly useful and yet took so long to evolve implies that they are also highly improbable.

Because so many key innovations are one-offs, it seems likely that  these are fluke events, low probability events that act like a series of chokepoints in evolutionary history. The emergence of intelligence isn’t like winning the lottery- it’s like Earth won the lottery not just once again, and again, and again, and again. On other worlds, such critical adaptations might evolved too late for intelligence to emerge before the sun went nova— or not at all.  Let’s say there are seven innovations that are both critical and unlikely- the origin of life, photosynthesis, complex cells, sex, complex animals, skeletons, and intelligence, each with a 10% chance of evolving. The odds of an Earthlike planet evolving intelligence would be one in 10 million. But such adaptations could be even more improbable- photosynthesis required a suite of innovations in proteins, pigments, and membranes. Similarly, bilaterian animals result from multiple innovations. So maybe such critical, complex adaptations evolve just 1% of the time. If so, intelligent life would appear on just one in 100 trillion worlds. And if highly habitable worlds are rare, we might be the only intelligent life in the galaxy— or the visible universe.

If an infinite number of monkeys hit the keys on a typewriter, one of them writes Hamlet. In the same way, given a seemingly infinite (or for all we know, actually infinite) number of planets, if natural selection plays around with DNA long enough, one of those planets is likely to have a string of lucky breaks and evolve intelligence. What might that world that look like? It would look an awful lot like Earth’s evolutionary history- a series of remarkable and unique innovations like photosynthesis and complex cells.

And yet, we’re here. That has to count for something, right? If evolution only gets lucky one in a trillion times, what are the odds we happen to be on the incredibly improbable planet where it actually happened? In fact, the odds of us being on that lucky world are 100%. Because obviously we couldn’t have this conversation on one of those other worlds that didn’t first evolve photosynthesis, complex cells, or animals. This is the anthropic principle: the history of Earth must have been such that intelligent life could evolve, otherwise we wouldn’t be here to wonder about it.

 

Do Mass Extinctions Increase Diversity, and if so, how?

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Over the past half-billion years, the Earth has been hit again and again by mass extinctions, wiping out most of the species on the planet. And yet every time, life recovered, and ultimately went on to increase in diversity. Is life just incredibly resilient, or is something else going on: could extinctions actually have helped life diversify and succeed? And if so, how?

Mass extinction is probably the most striking pattern seen in the fossil record. Vast numbers of species disappear, rapidly, simultaneously, and around the world. Driving extinction on this scale generally requires some kind of global environmental change, both so severe and so rapid that species disappear, rather than adapt to it. Most extinctions seem to be driven by catastrophes. Massive volcanic eruptions drove the extinctions at the end of the Devonian, Permian and Triassic. Severe glaciation drove the Ordivician-Silurian extinctions. One, the end-Cretaceous extinction of the dinosaurs, was driven by an asteroid. These “Big Five” extinctions get the most attention because, well, they’re the biggest, but other minor, but still civilization-ending events occurred as well, like the pulse of extinction that preceded the end-Permian event, and the Cenomanian-Turonian event that wiped out spinosaurs and carcharodontosaurs among the dinosaurs.

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These events were indescribably destructive. The Chicxulub asteroid impact that ended the Cretaceous shut down photosynthesis for years and led to decades of global cooling. Anything that couldn’t shelter from the cold, or find food in the darkness- which was most species- perished. Dinosaurs and pterosaurs went extinct, as well as most birds, mammals, lizards, snakes, plants, even many insects. Extinctions were equally severe in the sea, with plesiosaurs, mosasaurs, and ammonites disappearing; many invertebrates and even many plankton and other single-celled organisms, like foraminifera, were hit hard. Perhaps 90% or more of all species disappeared in just a few years.

But life bounced back, and the recoveries were rapid. Over 90% of mammal species were eliminated by the asteroid, but they recovered, and then some, within 300,00 years, then went on to evolve into horses, whales, bats, and our primate ancestors. Similarly rapid recoveries and radiations were seen in birds, lizards and fish. Much as the first finches to reach the Galapagos Islands diversified to occupy the vacant niches, the survivors of the extinction found a largely empty world, and diversified to fill it.

This pattern of recovery and diversification occurs after every mass extinction. The end-Permian extinction saw mammal-like species take a hit, but reptiles flourished afterward. When the reptiles were hit during the end-Triassic event, the surviving dinosaurs diversified and took over the planet. Although a mass extinction ended the dinosaurs, they only evolved because of mass extinction in the first place.

Despite this seeming chaos, life has slowly diversified over the past 500 million years. In fact, several observations hint that extinction might actually drive diversity. For one, the most rapid periods of diversity increase occur immediately after a mass extinction. But the recovery isn’t just driven by an increase in the number of species. During a recovery, animals innovate, finding new ways of making a living.

They exploit new habitats, new foods, new modes of locomotion. Our fishlike forebears first crawled onto land after the end-Devonian extinction. Giant dinosaurs, larger than anything that had existed before, appeared after the end-Triassic extinction. And when those dinosaurs then disappeared, whales took to the sea, bats to the sky, horses started eating plants, and our primate ancestors became specialized for life in the trees. Extinction doesn’t only drive speciation, it drives innovation. It’s not a coincidence that the biggest pulse of innovation in life’s history- the appearance of complex animals in Cambrian Explosion- occurs in the wake of the extinction of the Ediacaran animals that went before them.

Innovation may increase the number of species that can coexist because it allows species to move into new niches, instead of simply fighting over the old ones. Fish crawling onto land didn’t compete with fish in the seas; bats hunting in the dark with sonar didn’t compete with day-active birds. Innovation means diversity isn’t a zero-sum game; species can diversify without driving others extinct. So why does extinction drive innovation?

It may be that stable ecosystems hold back innovation. A modern wolf is probably a far more dangerous predator than a Velociraptor, but a tiny mammal couldn’t evolve into a wolf in the Cretaceous because there were velociraptors. Any experiments in carnivory would have ended badly, with unspecialized mammals competing with- or simply eaten by- the well-adapted Velociraptor.

But the extinction of that Velociraptor gives the mammal the freedom to experiment with new niches. Initially, mammals were poorly adapted to a predatory lifestyle, but without dinosaurs to compete with or eat them, they didn’t have to be very good to survive. They flourished in an ecological vacuum- ultimately becaming large, fast, intelligent pack hunters. In the lull after an extinction, evolution may be able to experiment with designs that are initially poorly adapted, but which have long-term potential. With the show’s stars gone, evolution’s understudies finally get their chance to prove themselves.

Life will even recover from the current wave of human-induced extinctions, perhaps reach even higher diversity given time. That’s not to justify complacency- it will take millions of years.

Economists talk about creative destruction, the idea that creating a new order drives the destruction of the old one. But evolution hints there’s another kind of creative destruction, where the breakdown of the old system creates a vacuum, and destruction actually precedes and drives the creation of something new, and often better.

This idea has relevance to human history. The extinction of the Pleistocene megafauna must have decimated hunter-gatherer bands, but it may have given farming a chance to develop. The Black Death produced untold human suffering, but the shakeup of political and economic systems may have led to the Renaissance.

That’s perhaps worth bearing in mind given our current political and social upheavals. The loss of something old creates opportunity for  new things to take root. Every cloud has a silver lining, even the debris cloud from an asteroid impact. It’s not just that there are opportunities when things are bad. It’s that when things are at their worst, is precisely when the opportunity is the greatest.

The Dinosaur Microbiome

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Dinosaur Bone in Dinosaur Provincial Park, with Estwing and Stetson

Life lives pretty much everywhere. In fact one of the most astonishing biological discoveries of the past century has been the discovery that the biosphere extends much further than we realized- into hot springs and hydrothermal vents, below Antarctic ice sheets… and miles underground. We tend to think of the subterranean world as being the realm of geologists, but in fact it’s the largest ecosystem on the planet- extending deep below the surface of the earth and the floor of the ocean.

 

So what does this have to do with paleontology? In recent years, we’ve seen some startling claims published about organic material preserved inside dinosaur bones- protein and protein-derived materials, DNA, even cells and tissues like blood vessels. Contamination is always an issue with these kinds of studies. Techniques designed to detect small amounts of DNA or protein from 75 million years ago must be incredibly sensitive- that also means that they’re sensitive to picking up biological material from you, the bacteria on your hands, a specimen in the lab, or what you had for lunch. As a result, rigorous protocols have been put in place to deal with contamination. But what if the contamination starts before the bone has even left the ground?

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Spotting a bone. Note the bright orange stuff- it’s a species of lichen.

Doing fieldwork in Dinosaur Provincial Park, in Alberta, I was always struck by the fact that a bright orange lichen sometimes grew on the bone- it was common enough you could actually use it to help find the bones in the field, like they were sprayed with dayglo orange paint. The lichen seemed to like the bone for some reason. For one thing, the bone was stable, whereas the muds and sandstones eroded away, so the lichen couldn’t grow there. It also had lots of moisture inside its spongy internal structure. And it was full of minerals, which a growing lichen needs. Living things actually used bone as a microhabitat. What’s more, when you peeled back the rock to expose the bone, you’d sometimes find sagebrush wrapped around it, probably feeding off the minerals. Which raises the question- why would we assume that bone is sterile? Why wouldn’t it be shot through with microbes- just the way the rocks around it are?

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Orange lichen growing on a dinosaur bone

In a new study, my colleagues and I (to be honest, almost entirely my colleagues) have been able to isolate DNA and amino acids from a dinosaur bone, dug up from Dinosaur Provincial Park in Alberta, in what must be one of the strangest dinosaur digs ever undertaken- since we weren’t hunting dinosaurs at all, but microbes living inside them. We excavated a bonebed of the dinosaur Centrosaurus, sterilizing tools with bleach, alcohol, and a blowtorch to prevent the introduction of contaminating bacteria, wrapping the bones in sterilized aluminum foil as soon as they were exposed, and then freezing them for later analysis in the lab. What we were able to show is that the organic material in these bones not ancient.
The DNA comes from bacteria. The most common thing is called Euzebya which is (don’t ask me why) found in odd places like Etruscan tombs and the skin of sea cucumbers. What’s more, the bacteria in the bone are at high densities- about ten times that of the surround rock. Bacteria don’t just work their way into the bone, they actively prefer living there.

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The hook off of a Centrosaurus frill. This bone isn’t dead, but home to a community of microbes. 

 

 

The amino acids also show biological activity, since they’re mostly left handed. If you remember intro biology, the chemistry of all living things has a handedness. The individual amino acids that make up proteins can be in either a left-handed form, or a mirror-image, right-handed form. Living things exclusively make and use left-handed amino acids, but after they die, the amino acids slowly flip their symmetry back and forth, to convert to a 50:50 mixture of left- and right-handed forms- a process called racemization. Racemization takes thousands, not millions of years. So if we have amino acids from dinosaurs living millions of years ago, they should be fully racemized, a 50:50 ratio of left and right handed amino acids. But since the amino acids we found are mostly of the left-handed variety, that means the biological material is mostly very recent, not from the Cretaceous.

We also carbon-dated the material. Our atmosphere is full of trace amounts of carbon 14, a highly radioactive element with a half-life of around 6,000 years- that is, half of it undergoes radioactive decay and breaks down every 6,000 years. When plants and other organisms use C02 to build things, they (and anything they eat) incorporate that c 14. After death, they stop taking in new carbon 14, and the amount left halves every 6000 years or so. The upshot is that we can figure out how old stuff is by the amount of C 14 remaining- up until a limit of 50,000 years, at which point its basically all gone. Since dinosaur bones are more than 50,000 years old, any carbon 14 from the dinosaur era incorporated into DNA and proteins would have long ago disappeared. But we found a lot of carbon 14- meaning most or all of the material we isolated wasn’t from dinosaurs, but from recently living things.

All this points in the same direction- the bones are in a very real sense alive. They host a thriving bacterial community, a microbiome, and the amino acids and proteins we isolated were primarily recent. Some of the material is a bit older- there are some right-handed amino acids, and the carbon 14 ratios are a bit lower than you’d expect on a modern living thing- so it does seem that there are some older organics in there. But whether they are thousands of years old, or millions, we don’t know.

What does this mean for dinosaur DNA and proteins? Well, it’s too soon to say, but it’s one more reason for skepticism. Currently, the record for the preservation of DNA is a genome extracted from a 750,000 year old horse. DNA has a relatively short half-life, about half the DNA breaks down every 500 years, so it’s thought to become unreadable after about 1.5 million years even under optimal conditions. It’s extremely unlikely we can get Jurassic DNA from 150 million years ago, or even DNA from the much younger T. rex, which went extinct a mere 66 million years ago.
Proteins appear to be more robust than DNA, however, protein also tends to break down over time, with water molecules breaking apart the bonds between amino acids. The oldest confirmed proteins are about five times older than the oldest DNA: protein sequences from ostrich eggshell have been recovered going back almost 4 million years. Which is not to say that organics can’t survive from dinosaurs- we’ve been able to isolate pigments, for example- but we need to be a little skeptical. How and whether things like cells and vessels can survive that long isn’t entirely clear.

Clearly some organics do survive- actually a lot of stuff. Sporopollenin, for example, is a biopolymer that makes up the wall of pollens and it is an incredibly robust molecule. It’s nigh-invulnerable, to the point that you can dissolve rock in concentrated acids and all that remains are little 75 million year old pollen grains from the Cretaceous. So we can get pollen grains from dinosaur age sediments. We can also recover organic-walled fossils like dinoflagellates.

Personally, I’m mostly a morphologist- I don’t have the expertise to critically evaluate finer points of debates about organic chemistry. But I can say that extraordinary claims require extraordinary evidence. The claims of DNA and protein isolated from dinosaurs are far, far older than anything else. Importantly, these finds have been difficult to replicate. And there’s a lot of contamination down there. That some kind of heavily altered protein- basically fossil protein– might remain seems more plausible. I’m not saying it doesn’t exist. I don’t know. But there’s a lot of other stuff down in the subsurface community- not just bacteria, but archaeans, eukaryotes, multicellular organisms like fungi, nematodes. Before we identify ancient traces, we need to be able to rule out contamination, and contamination is going to be everywhere. But by getting a better handle on what that contamination looks like, we can at least do a better job telling the contamination from the original organics.

But  I also think the microbiome is fascinating in its own right, not just an annoying contaminant. It’s interesting to think about what it says about the evolution of life on our planet. It’s bizarre to think, but dinosaurs didn’t stop interacting with the environment when they died, they just did so in a different kind of way. After that Centrosaurus died, it was food for scavengers- tyrannosaurs, dromaeosaurs, pterosaurs, beetles and flies. Then the skeleton was buried underground, and bacteria continued to eat away at the proteins. After thousands of years, it was part of the subsurface microbiome. Then the Western Interior Seaway rolled in, and it was part of a deeper microbiome existing under the bottom of the ocean. Then it rolled back out again after the dinosaurs died out, thousands of feet of sediments were piled up, and the Centrosaurus bone was part of a deep biome, far underground. At last, the glaciers came through, ground the land down, and an ancient glacial lake burst, gouging out the badlands that would become Dinosaur Provincial Park. From the time that Centrosaurus died up until now, it has seen a succession of microbial communities- on a Cretaceous floodplain, then underneath it, then under a sea, then under a glacier, and at last, just below the prairies of Alberta. It’s amazing to think that the life of the past still affects the life of today- as Faulkner put it, the past isn’t dead, it isn’t even past.

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A piece of fossil wood hosts a diverse community of lichens. 

There’s a good writeup of the paper here in the Atlantic by Ed Yong.

The original paper is here: Saitta, E.T., Liang, R., Lau, M.C., Brown, C.M., Longrich, N.R., Kaye, T.G., Novak, B.J., Salzberg, S.L., Norell, M.A., Abbott, G.D. and Dickinson, M.R., 2019. Cretaceous dinosaur bone contains recent organic material and provides an environment conducive to microbial communities. eLife8, p.e46205.