Life converges

One of the most fascinating features of biological evolution -- particularly as it applies to the possibility of life on other planets -- has to do with the concept of constraint.

Which features of life on Earth are, in some sense, inevitable?  Are there characteristics of terrestrial organisms that we might expect to find on any inhabitable world?  Stephen Jay Gould looked at this question in his essay "Replaying the Tape," from his brilliant book on the Cambrian Explosion, Wonderful Life:

You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past -– say, to the seas of the Burgess Shale.  Then let the tape run again and see if the repetition looks at all like the original.  If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur.  But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life?  What could we then say about the predictability of self-conscious intelligence?  or of mammals?

Some features that have been suggested as evolutionarily constrained, with arguments of varying levels of persuasiveness, are:

• a genetic code based on some kind of nucleic acid (DNA or RNA, or some chemical analogue)
• internal cell membranes made of phospholipids, to segregate competing chemical reactions from each other 
• multicellularity, with some level of tissue specialization
• in more complex organisms, some form of symmetry, with symmetrically-placed organs
• some kind of rapid-transit system for messages, analogous to our nervous system (but perhaps not structured the same way)
• cephalization -- concentration of the central processing centers and sensory organs near the head end

It's interesting when science fiction tackles this issue -- and sometimes comes up with possible pathways for evolution that don't result in humanoids with strangely-shaped ears and odd facial protuberances.  A few that come to mind are Star Trek's silicon-based Horta from the episode"Devil in the Dark," the blood-drinking fog creature from "Obsession," the giant single-celled neural parasites from "Operation Annihilate," and Doctor Who's Vashta Nerada, Not-Things, Gelth, and Midnight Entity.

So the search for extraterrestrial life requires we consider looking not only for "life as we know it, Jim," but life as we don't know it.  Or, more accurately, to consider to what extent our terrestrial biases might be blinding us to the possibility of what evolution could create.

It's worth considering, however, how often evolution here on Earth ends up landing on the same solutions to the problems of survival and reproduction over and over again, a phenomenon called convergent evolution.  Eyes, or analogous light receptor organs, have evolved multiple times -- some biologists have suggested as many as fifty different independent lineages that evolved some form of eye.  Wings occurred separately in four groups of animals -- birds, pterosaurs, insects, and bats.  (If you include structures for gliding, add flying squirrels, sugar gliders, colugos, flying fish, and flying lizards.)

Even biochemical pathways can reappear, something I find astonishing.  Take, for example, the research that came out this week in Nature Chemical Biology, which found that two only distantly-related plants -- ipecac (Carapichea ipecacuanha), in the gentian family, and sage-leaved alangium (Alangium salviifolium), in the dogwood family, have both come up with complex biochemical pathways to generate the same set of bitter, emetic compounds -- ipecacuanha alkaloids.

The last common ancestor of these two species was over a hundred million years ago, so there's a strong argument that they evolved this capacity independently.  And indeed, when the biochemists looked at the enzymatic pathways, they're different -- they found entirely different chemical synthesis methods for producing the same set of end products.  Weirdest of all, they both evolved an enzyme that cleaves a sugar molecule from the alkaloid precursor, and that's what activates it (i.e., makes it toxic).  In the living plant's tissues, the enzyme and the precursor are segregated from each other.  It's only when they're brought together -- such as when a herbivore chomps on the leaves -- that the sugar is split away from the precursor, the alkaloid is activated, and the herbivore starts puking its guts up.

Clever strategy.  So clever, in fact, that it was stumbled upon by two entirely separate lineages of plants.  The rules organisms play by are the same, so perhaps not surprising there are similar outcomes sometimes.

The whole thing highlights the fact that there is a limited range of solutions for the fundamental difficulties of existence.  It has to make you wonder if, when we do find life elsewhere in the universe, it might look a lot more familiar that we're expecting.  I don't think it's likely we'll bump into Romulans or Ice Warriors or Krillitane, but maybe there are features of life on Earth that will re-evolve in just about any conceivable habitable planet.

But hopefully there won't be any Vashta Nerada.  Those things are terrifying.

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Taking flight

One of the many things I find fascinating about the evolutionary model is how different lineages can happen on the same "solution" to the problems of surviving and reproducing, leading to similarities cropping up that don't result from common ancestry.  This is a phenomenon called convergent evolution, and explains why the North American flying squirrel and Australian sugar glider look a lot alike, even though they are only distantly related.  (The flying squirrel is a rodent, and the sugar glider a marsupial more closely related to kangaroos.)

I put the word "solution" in quotes and use it with caution, because this makes it sound like evolution is forward-looking, which it is not.  As Richard Dawkins explains brilliantly in his book The Blind Watchmaker, to trigger evolution, all you have to have is an imperfect replicator (in this case, DNA) and a selecting agent.  To phrase it more like Darwin would have put it: variation coupled with differences in survival rate.

I recall how surprised I was to learn that the eye has actually evolved multiple times.  Starting with light-sensitive spots, such as you still find today in many microorganisms, variations on different lineages came up with a variety of different "solutions" -- the pinhole-camera eye of a chambered nautilus, the cup-shaped eye of a flatworm, the compound eye of a fly, and our own eye with a transparent lens like that of a refracting telescope.  All these adaptations work just fine for the animal that has them.  (Eye formation in a number of species is controlled by the paired-box 6 [PAX6] gene, without which eyes won't form at all.  It's such a critical gene that it is conserved across thousands of species -- in fact, your PAX6 gene and a mouse's are identical, base-pair-for-base-pair.)

The reason this subject comes up is because of some research published in the journal Current Biology that showed another trait -- flight -- not only evolved separately in groups like insects and birds, but even in the dinosaurian ancestors of today's birds, it evolved more than once.

A team led by paleontologist Rui Pei of the Chinese Academy of Sciences analyzed bone and feather structure of various dinosaur groups to see if they flew, glided, or were using their feathers for a different purpose (such as keeping warm).  To their surprise, it was found that multiple lineages were capable of flying, or nearly so.  The authors write:

We [used] an ancestral state reconstruction analysis calculating maximum and minimum estimates of two proxies of powered flight potential—wing loading and specific lift.  These results confirm powered flight potential in early birds but its rarity among the ancestors of the closest avialan relatives (select unenlagiine and microraptorine dromaeosaurids).  For the first time, we find a broad range of these ancestors neared the wing loading and specific lift thresholds indicative of powered flight potential.  This suggests there was greater experimentation with wing-assisted locomotion before theropod flight evolved than previously appreciated.  This study adds invaluable support for multiple origins of powered flight potential in theropods (≥3 times), which we now know was from ancestors already nearing associated thresholds, and provides a framework for its further study.

Here are their results, in graphical form:

As you can see, actual birds -- labeled "Later-diverging avialans" near the bottom of the tree -- were far from the only ones to have flight capability.  Rahonavis, Microraptor, and several of the anchiornithines were probably fliers, and only the last mentioned is on the same clade as today's birds.

Flying is pretty useful, so it's no wonder that when feathers evolved from scales -- probably, as I mentioned earlier, in the context of warmth and insulation -- it was only a small step remaining toward lengthening those feathers to the point that their owners could catch a breeze and glide.  After that, the same kind of refinement took over that happened with the eye, and eventually, you have true flight.

So that's yet another cool bit of research about prehistory.  Wouldn't you like to know what those prehistoric fliers looked like?  I'd love to see them.  From a distance, because a lot of them were predators.  For example, Microraptor is Greek for "tiny hunter," and were a little like miniature velociraptors with wings.

If you wanted an image to haunt your dreams.

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Very like a mammal

"Prior to the End-Cretaceous Extinction, mammals were all small and shrew-like, restricted to skulking, scurrying forms because of competition from, and predation by, dinosaurs.  Once the dinosaurs were out of the way, the mammals were free to diversify and to grow larger."

How many times have we all heard this?  And it certainly sounds plausible; being large and obvious when there were hungry carnivores like Velociraptor around seems like a good way to be turned into dinner.

But the fossil record shows that the truth is more complicated -- and far more interesting.

Take, for example, Castorocauda lutrasimilis.  This animal was around fifty centimeters long and weighed in at around three-quarters of a kilogram.  It was sleek, streamlined, with a bullet-shaped head, a fine pelt of soft fur, and a flat, paddle-like tail.  Here's an artist's reconstruction:

[Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Castorocauda BW, CC BY 3.0]

If you're reminded of something like a beaver or an otter, you're not alone; the scientific name means "beaver's tail and looks like an otter."  Surprisingly, it was closely related to neither one; in fact, it's not even a true mammal, but a docodont, which split off from other mammal-like forms (including our own ancestors) way back in the early Jurassic period -- while there were plenty of dinosaurs lumbering around the place.

The docodonts, and a handful of other groups of Mesozoic cousins to mammals, are mostly known from the exceptional fossil beds of the Tiaojishan Formation in northern China, where paleontologists have found a wealth of mid- to late-Jurassic fossils of mammaliaformes -- as they call Mesozoic mammals and their near relatives.  And amongst those fossils they not only find otter-like aquatic species, but ones that have adaptations an awful lot like moles, squirrels, and possums.

This adds another cluster to the list of cool examples of convergent evolution, where two only distantly-related species evolve to resemble each other superficially because of similar selective pressures.  (A famous modern pair is the North American flying squirrel and the Australian sugar glider; at a quick glance these two look very much alike, but a closer examination would show that they're not even in the same order.  The flying squirrel is a rodent, and the sugar glider a marsupial.)

The docodonts and other side branches of the mammaliaformes all disappeared by the middle of the Cretaceous Period, replaced by true mammals including multituberculates, monotremes, marsupials, and placentals.  Why this happened isn't certain; given that we know the non-mammal mammaliaformes from only a few isolated geological strata, our information on them is limited.  We do know, however, that the mammals who survived were mostly "small and shrew-like," so there's a grain of truth to the old model.

What's most fascinating is that after the End-Cretaceous Extinction, these survivors re-diversified, and "re-invented" a bunch of the adaptations the docodonts had a hundred million years earlier.  This has interesting implications, not only for the evolution of life on Earth but for the kinds of living things we might expect to find on other planets.  It's long been a fascinating question to me to what extent evolution is constrained -- what limitations there are on natural selection that might result in its generating the same patterns over and over because those are the features that work best in pretty much any environment.  There are a few that seem likely, such as having the main sensory organs near the mouth and at the anterior of the body; I'd expect those to be frequent no matter where you go.

But what Castorocauda and the other docodonts show is that other sorts of traits can repeat, too.  After all, there are only so many ways you can move around, find food, find shelter, avoid being eaten, and regulate your own body temperature.  It might be surprising at first that the otter-like Castorocauda (and the possum-like Borealestes and the squirrel-like Shenshou) "re-evolved" (as it were) over a hundred million years later, but it suggests that making a living requires the same toolkit pretty much regardless.  

So maybe when we find life on another planet, it'll be far more familiar than we expect -- and that "life as we know it, Jim" might be there to greet us when we arrive.

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Drawn together

Convergent evolution occurs when only distantly-related species are under the same selective pressures, and evolve to look alike.  A particularly good example of this is the North American flying squirrel (a rodent) and the Australian sugar glider (a marsupial).  Put them side-by-side, and they're hard to tell apart, and both have the distinctive kite-like flap of skin between the forelegs and hind legs, allowing them to catch a breeze and glide from tree to tree.

It's important to emphasize that while convergent evolution can result in organisms being similar in appearance or habits, it doesn't ever cause them to fuse into a single species.  Flying squirrels and sugar gliders maintain major differences in their genetic make-up, skeleton, dentition, and so on -- so however close the resemblance, they're still two separate species.

Convergence is actually fairly common in the natural world, which is why appearance is such a poor guide to determining who is related to whom.  There are only so many solutions to the problems posed by living in a particular environment, so it's inevitable that different lineages will happen on the same ones.  Flying, for example, has evolved independently at least four times -- birds, bats, pterodactyloids, and insects.  The structure and mechanics is different in each, which is indicative that they were independent innovations.

I was thinking about convergent evolution this morning as I read a paper in the journal Geodiversitas about the discovery of a remarkable fossil in Colombia.  It's the best-preserved and most complete skeleton ever found of Anachlysictis gracilis, a Miocene apex predator that belonged to a group called the sparassodontids.  (The name comes from the Greek σπαράσσειν, to tear to pieces, and ὀδόντος, tooth -- an indicator of how scary these animals were.)

Here's a photograph of the skeleton:

[Image courtesy of Daniella Carvalho and Aldo Benites-Palomino]

My guess is that looking at this, you're immediately reminded of the saber-toothed cats such as the famous Smilodon, which also were around during the Miocene Epoch but reached their pinnacle a few million years later, during the Pleistocene.  Surprisingly, this parallels my earlier example of the flying squirrel and sugar glider -- the saber-toothed cats were true felids, and thus placental mammals, while Anachlysictus and the other sparassodonts were marsupials.  The two species were drawn together by the forces of convergent evolution.  If you're a predator, having big nasty pointy teeth is a pretty good adaptation regardless what taxonomic group you belong to.

These striking carnivores were present in South America during what is called the "splendid isolation," prior to the tectonic shift that formed the Isthmus of Panama and allowed for the Pliocene Great Biotic Interchange.  South America had developed a unique biota, including not only the sparassodonts but a variety of other marsupial groups, most of which are now extinct.  Even the South American placentals didn't do so well, and were outcompeted (or hunted to death) by North American migrants.  Not long after the formation of Central America, a great many of the South American groups -- not only the sparassodonts, but the glyptodonts, litopterns, astrapotheres, pyrotheres, and xenungulates -- were gone forever.

The new fossil discovery will allow paleontologists to make some deductions about not only its anatomy, but its behavior. "In a future study we will address all the other bones in its body, which include various sections of the spine, ribs, hip, scapulae -- what we call 'shoulder blades' for humans -- and bones in its legs," said Catalina Suarez, of the Argentine Institute of Nivology, Glaciology and Environmental Sciences, who led the research team.  "This will allow us to explore aspects of how it moved, the position in which its neck held its head, whether it was a runner, whether it could climb, whether its hands could hold objects more easily, as many marsupials do when feeding, or whether it was a bit more difficult, as it is for example for a dog or a cat."

It's fascinating to learn more about these long-extinct animals, whose ecological role would be taken over by predatory placental mammals like wolves and the various big cats.  Even if they're extinct, their bones still have a story to tell -- of a saber-toothed marsupial who hunted in the forests of Colombia thirteen million years ago.

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A new view of the "eye lizard"

I am forever astonished at the level of detail we can infer from fossils that are hundreds of millions of years old.

The most recent example of this came from analysis of a fossil of Stenopterygius, an ichthyosaur that lived during the Jurassic Period (this particular fossil has been dated to about 180 million years ago).  We usually think of fossils as preserving bones and teeth, and occasionally impressions of scales or skin or feathers -- but this one was so finely preserved that researchers have been able to make some shrewd inferences about color, metabolism, and the structure of soft tissues.

Artist's conception of Stenopterygius [Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Stenopterygius BW, CC BY-SA 3.0]

We've known for a long time that ichthyosaurs are bizarre animals. They were streamlined predators that look remarkably like dolphins, although they are only distantly related (making the two groups a great example of convergent evolution).  A number of them had an even stranger feature, which is the largest eye-diameter-to-body-size ratio of any animal known -- the well-named Ophthalmosaurus (Greek for "eye lizard") was six meters long and had eyes the size of basketballs.

Stenopterygius was a bit smaller, with an average adult size of four meters.  But up until recently, all we've been able to do is speculate on what it might have looked like, and how it behaved.  A discovery in Germany, described in a paper in Nature called "Soft-Tissue Evidence for Homeothermy and Crypsis in a Jurassic Ichthyosaur" and authored by no fewer than 23 scientists, has given us incredibly detailed information on these oddball dinosaurs.

The authors write:

Ichthyosaurs are extinct marine reptiles that display a notable external similarity to modern toothed whales.  Here we show that this resemblance is more than skin deep.  We apply a multidisciplinary experimental approach to characterize the cellular and molecular composition of integumental tissues in an exceptionally preserved specimen of the Early Jurassic ichthyosaur Stenopterygius.  Our analyses recovered still-flexible remnants of the original scaleless skin, which comprises morphologically distinct epidermal and dermal layers.  These are underlain by insulating blubber that would have augmented streamlining, buoyancy and homeothermy.  Additionally, we identify endogenous proteinaceous and lipid constituents, together with keratinocytes and branched melanophores that contain eumelanin pigment.  Distributional variation of melanophores across the body suggests countershading, possibly enhanced by physiological adjustments of colour to enable photoprotection, concealment and/or thermoregulation.  Convergence of ichthyosaurs with extant marine amniotes thus extends to the ultrastructural and molecular levels, reflecting the omnipresent constraints of their shared adaptation to pelagic life.

So from a 180-million-year-old fossil, we now know that Stenopterygius (1) was a homeotherm (colloquially called "warm-blooded"), (2) had a blubber layer much like modern dolphins and whales, and (3) were countershaded -- dark on top and light underneath, to aid camouflage -- similar to dozens of species of modern fish.

This level of preservation is extremely unusual.  "Both the contour of the body and the remains of internal organs are clearly visible," said paleontologist Johan Lindgren of the University of Lund, who co-authored the paper.  "Surprisingly, the fossil is so well preserved that it is possible to observe individual cell layers inside the skin."

"This is the first direct chemical evidence of warm blood in an ichthyosaur, because a subcutaneous fat layer is a characteristic of warm-blooded animals," said Mary Schweitzer of North Carolina State University, also a co-author.  "Ichthyosaurs are interesting because they have many features in common with dolphins, but they are not related at all to these mammals that inhabit the sea.  But the enigma does not stop there...  They have many characteristics in common with living marine reptiles, such as sea turtles; but we know from the fossil record that they gave live birth to their young...  This study reveals some of those biological mysteries."

Which is pretty astonishing.  I've always had a fascination for the prehistoric world, and have spent more time than I like to admit wondering what it might have been like to live in the Jurassic world. This research gives us one more piece of information -- about a fierce prehistoric predator that shared some amazing similarities to creatures that still swim in our oceans.

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Taking flight

One of the many things I find fascinating about the evolutionary model is how different lineages can happen on the same "solution" to the problems of surviving and reproducing, leading to similarities cropping up that don't result from common ancestry.  This is a phenomenon called convergent evolution, and explains why the North American flying squirrel and Australian sugar glider look a lot alike, even though they are only distantly related.  (The flying squirrel is a rodent, and the sugar glider a marsupial more closely related to kangaroos.)

I put the word "solution" in quotes and use it with caution, because this makes it sound like evolution is forward-looking, which it is not.  As Richard Dawkins explains brilliantly in his book The Blind Watchmaker, to trigger evolution, all you have to have is an imperfect replicator (in this case, DNA) and a selecting agent.  To phrase it more like Darwin would have put it: variation coupled with differences in survival rate.

I recall how surprised I was to learn that the eye had actually evolved multiple times.  Starting with light-sensitive spots, such as you still find today in many microorganisms, variations on different lineages came up with a variety of different "solutions" -- the pinhole-camera eye of a chambered nautilus, the cup-shaped eye of a flatworm, the compound eye of a fly, and our own eye with a transparent lens like that of a refracting telescope.  All these adaptations work just fine for the animal that has them.  (Eye formation in a number of species is controlled by the paired-box 6 [PAX6] gene, without which eyes won't form at all.  It's such a critical gene that it is conserved across thousands of species -- in fact, your PAX6 gene and a mouse's are identical, base-pair-for-base-pair.)

The reason this subject comes up is because of some research published in the journal Current Biology last week that showed another trait -- flight -- not only evolved separately in groups like insects and birds, but even in the dinosaurian ancestors of today's birds, it evolved more than once.

A team led by paleontologist Rui Pei of the Chinese Academy of Sciences analyzed bone and feather structure of various dinosaur groups to see if they flew, glided, or were using their feathers for a different purpose (such as keeping warm).  To their surprise, it was found that multiple lineages were capable of flying or nearly so.  The authors write:

We [used] an ancestral state reconstruction analysis calculating maximum and minimum estimates of two proxies of powered flight potential—wing loading and specific lift.  These results confirm powered flight potential in early birds but its rarity among the ancestors of the closest avialan relatives (select unenlagiine and microraptorine dromaeosaurids).  For the first time, we find a broad range of these ancestors neared the wing loading and specific lift thresholds indicative of powered flight potential.  This suggests there was greater experimentation with wing-assisted locomotion before theropod flight evolved than previously appreciated.  This study adds invaluable support for multiple origins of powered flight potential in theropods (≥3 times), which we now know was from ancestors already nearing associated thresholds, and provides a framework for its further study.

Here are their results, in graphical form:

As you can see, actual birds -- labeled "Later-diverging avialans" near the bottom of the tree -- were far from the only ones to have flight capability.  Rahonavis, Microraptor, and several of the anchiornithines were probably fliers, and only the last mentioned is on the same clade as today's birds.

Flying is pretty useful, so it's no wonder that when feathers evolved from scales -- probably, as I mentioned earlier, in the context of warmth and insulation -- it was only a small step remaining toward lengthening those feathers to the point that their owners could catch a breeze and glide.  After that, the same kind of refinement took over that happened with the eye, and eventually, you have true flight.

So that's yet another cool bit of research about prehistory.  Wouldn't you like to know what those prehistoric fliers looked like?  I'd love to see them.  From a distance, because a lot of them were predators.  For example, Microraptor is Greek for "tiny hunter," and were a little like miniature velociraptors with wings.

If you wanted an image to haunt your dreams.

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This week's Skeptophilia book recommendation of the week is by the brilliant Dutch animal behaviorist Frans de Waal, whose work with capuchin monkeys and chimps has elucidated not only their behavior, but the origins of a lot of our own.  (For a taste of his work, watch the brilliant TED talk he did called "Moral Behavior in Animals.")

In his book Mama's Last Hug: Animal Emotions and What They Tell Us About Ourselves, de Waal looks at this topic in more detail, telling riveting stories about the emotions animals experience, and showing that their inner world is more like ours than we usually realize.  Our feelings of love, hate, jealousy, empathy, disgust, fear, and joy are not unique to humans, but have their roots in our distant ancestry -- and are shared by many, if not most, mammalian species.

If you're interested in animal behavior, Mama's Last Hug is a must-read.  In it, you'll find out that non-human animals have a rich emotional life, and one that resembles our own to a startling degree.  In looking at other animals, we are holding up a mirror to ourselves.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Look at that shine!

There's this bird called the cassowary, have you heard of it?

I think a better name for it would be the "Giant Blue-headed Australian Death Turkey."  They're ungainly-looking things, but (1) they're big, and (2) they're fast.  An adult GBADT can be two meters tall and weight 55 kilograms.  Not only that, but if they feel threatened, they don't run or fly away as any normal species of bird would do.  No, this is Australia.  What they do is run toward people, jump up, and kick them with razor-sharp talons, attempting -- sometimes successfully -- to disembowel them.

Think I'm joking?  This is an actual (i.e. un-Photoshopped) photograph of a guy trying to avoid being killed by a furious cassowary.

The reason this comes up is that cassowaries have another strange feature besides being, essentially, emus with daggers strapped to their feet.  Their black feathers have the quality of iridescence -- something you might not notice if it was leaping at you -- but from a safe distance, their feathers have an oily rainbow sheen.

This is more than just simple pigmentation.  The structures in the feathers containing the black pigment are called melanosomes, and they come in a variety of shapes and sizes in different species.  The brightly-colored throat patches ("gorgets") in hummingbirds are the color they are because of melanosomes.

But if the pigment they contain is black, how do hummingbirds display their amazing array of colors, and how do cassowaries gain their sheen?

The reason is a phenomenon called optical interference, and has to do with the multiple clear layers of keratin that separate the layers of melanosomes.  Light passing through those clear layers is refracted, and crosses light waves refracted by other layers -- and because of this, some wavelengths of light undergo destructive interference (they cancel each other out) and others constructive interference (they reinforce each other).  In our local Ruby-throated Hummingbirds, the keratin layers are spaced so the wavelengths that reinforce are ones that our eyes see as being in the red region of the spectrum; other colors get cancelled out.  Thus, the ruby throat of the Ruby-throat.

But change the spacing of the layers, and you change what colors reinforce.  So you can get the Violet-tailed Sylph of Ecuador...

[Image licensed under the Creative Commons Joseph C Boone, Violet-tailed Sylph 2 JCB, CC BY-SA 4.0]

... the aptly-named Magnificent Hummingbird of Mexico, Central America, and southern Arizona...

[Image licensed under the Creative Commons Don Faulkner, Magnificent Hummingbird (7047734993), CC BY-SA 2.0]

... and over two hundred others, each with its own different spacing of the keratin layers in the feathers, and thus, each with its own array of spectacular, iridescent colors.

What's fascinating about this evolutionarily is that cassowaries and hummingbirds have been separate lineages for a long time.  Their last common ancestor is estimated at eighty million years ago, so predating the extinction of the non-avian dinosaurs by a good fourteen million years.  And some birds don't have this kind of iridescence -- their feather colors come from ordinary pigments, not a lot different than different colors of paint.  So how did two widely-separated groups of birds end up landing on the same solution for being colorful?

It's a very striking example of convergent evolution, where different organisms end up becoming superficially similar (usually only on one or two traits) because of similar selective pressures.  And apparently the innovation came about a long time ago in both lineages, as I found out in a paper this week in Science Advances that details information about some fossil feathers from relatives of the cassowary that were around 52 million years ago, during the Eocene Epoch.

In "Cassowary Gloss and a Novel Form of Structural Color in Birds," by Chad Eliason and Julia Clarke of the University of Texas - Austin, we read about an incredibly detailed analysis of feather fossils from the Green River Formation in Wyoming.  Using an electron microscope, the authors were able to measure the spacing of the melanosome layers and keratin layers, and determined that the species the feathers came from -- the lithornithid Calxavis (or Calciavis) grandei -- was black, with a deep iridescence on the wings.

The idea that we could actually find out what color an extinct species was using its fossilized feathers is amazing.  When I look at "artist's reconstructions" of prehistoric animals, I have to remind myself constantly that all the colors are just guesses based on analogies (sometimes incorrect ones) to modern species.  But now we actually have a pretty good idea of what a bird looked like who last flew around fifty-some-odd million years ago, which is kind of mind-boggling.

You have to wonder what other characteristics Calxavis shared with its modern cousins.  Unfortunately, we still know next to nothing about the behavior of long-extinct animals, so more than likely we'll never have anything more than guesses about how it acted when it was alive.

Who knows, maybe it even rushed at prehistoric predators and tried to rip them apart with its talons.  I mean, the Giant Blue-headed Australian Death Turkey's bad attitude has to come from somewhere.

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This week's Skeptophilia book-of-the-week is one that should be a must-read for everyone -- not only for the New Yorkers suggested by the title.  Unusual, though, in that this one isn't our usual non-fiction selection.  New York 2140, by Kim Stanley Robinson, is novel that takes a chilling look at what New York City might look like 120 years from now if climate change is left unchecked.

Its predictions are not alarmism.  Robinson made them using the latest climate models, which (if anything) have proven to be conservative.  She then fits into that setting -- a city where the streets are Venice-like canals, where the subways are underground rivers, where low-lying areas have disappeared completely under the rising tides of the Atlantic Ocean -- a society that is trying its best to cope.

New York 2140 isn't just a gripping read, it's a frighteningly clear-eyed vision of where we're heading.  Read it, and find out why The Guardian called it "a towering novel about a genuinely grave threat to civilisation."

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Hey, Rocky! Watch me pull a rabbit out of a hat!

Because so much of the world lately seems to be immersed in hatred, violence, and just general suckiness, for today I'm going to retreat to my Happy Place, which is: cool scientific discoveries.

Today's contribution from the Happy Place comes from the fields of paleontology and evolutionary biology, two disciplines that are near and dear to my heart.  While my educational background is kind of all over the map (less charitable sorts have called it "a light year across and an inch deep"), evolutionary biology has been something of a passion of mine for ages.  In getting my teaching certificate, I did as many courses as I could that focused on such things as population genetics, cladistics, and the origins of life, so I have come to think of that as being more or less my specialty within the field.

The discovery that spurred today's post comes from northeastern China, where two new species of mammal were uncovered (literally and figuratively) -- Maiopatagium and Vilevolodon.  These species were probably closely related, and appear to have been small tree-dwellers who had flaps of skin that ran from the outsides of the front legs to the outsides of the hind legs, so that when necessary, they could jump from a tree branch, fling their limbs outward, and glide like a living kite.

[image courtesy of study leader Zhe-Xi Luo of the University of Chicago]

If you're thinking, "Wait a second.  Isn't that just a flying squirrel?", you should know two things: (1) these two species were only very distantly related to flying squirrels and other rodents; and (2) they were alive during the Jurassic Period -- something on the order of 160 million years ago, when the dominant life forms were dinosaurs.  (For comparison purposes, the earliest known rodents didn't show up until 100 million years later.)  They belong to a group called Euharamiyids, one of the four branches of true mammals (the other three are the Multituberculates, which like the Euharamiyids are extinct; the Monotremes, which include egg-laying mammals such as the platypus; and the Therians, which encompass all other mammals, including us).

What I think is coolest about all of this -- besides the fact that ancient animals are simply inherently cool -- is that it's further evidence of the fact that similar selective pressures often result in separate lineages that happen upon the same "solutions" to evolutionary problems.  This is called convergent or parallel evolution, and one of the best examples of this is the evolution of flight and/or gliding.  Taking to the air has apparently evolved over and over again, resulting in the most familiar flying groups -- birds, insects, and bats -- but also in...

Pterodactyloids:

Colugos:

Flying fish:

Sugar gliders:

and the aforementioned flying squirrels:

... the latter of which were studied extensively by noted scientists Boris Badinov and Natasha Fatale.

And now, we can add two more to the list, a pair which (like all the rest) evolved aerobatics completely independently of all the others.

Anyhow, the whole thing illustrates a fundamental rule of biology, which is that there are a limited number of powerful evolutionary drivers (the most important being finding food, not getting turned into food, avoiding the vagaries of the environment you're in, and finding a mate), and a limited number of solutions to those drivers in the real world.  So it's inevitable that the same kinds of structures and behaviors will evolve over and over, even in groups that aren't very closely related.  What is most remarkable about this particular discovery, however, is how early the innovation of gliding in mammals evolved -- back when the whole mammalian clade had barely gotten started, and the dinosaurs still had 95 million years left before a giant meteor strike ended their hegemony.

And all of this ties into another field I'm fascinated with, which is exobiology -- the study of alien species.  At the moment, the number of available samples to study is zero, so we're left speculating based on what we have here on our home planet.  But the fact that we see the same sorts of patterns cropping up again and again -- bilateral symmetry, organs for sensing light and sound, defensive and offensive weapons, and adaptations for rapid locomotion -- is a pretty sound argument that when we do come across life on other planets, it will probably have some striking similarities to what we see here on Earth.

So that's our cool scientific discovery of the day, courtesy of a research team working in China.  And unfortunately I need to wrap up this post, which means I have to leave my Scientific Happy Place and return to the real world, at least for a little while.  Maybe I'll luck out and there'll be other fun and fascinating discoveries announced soon so that I can read something other than the news, which is more and more making me wonder if it might not be time for another giant meteor to press "reset" on the whole shebang.