Fishy business

My evolutionary biology professor told our class, many years ago, "The only reason we came up with the word species is because humans have no near relatives."

It's a comment that has stuck with me.  We perceive species as being these little cubbyholes with impenetrable sides, and once you've filed something there, it stays put.  Of course a polar bear and a grizzly bear are different species.  How could they be otherwise?

But when you start pushing at the definition a little, you find that it gives way almost immediately.  Ask some non-scientist how they know polar bears and grizzly bears are different species, and you'll likely get an answer like, "Because they look completely different."  And, to be fair, that's more or less how the father of taxonomy, Carl Linnaeus, did it.

Problems creep in almost immediately, though.  The "of course different species" polar and grizzly bears look far more alike than do, say, a chihuahua and a St. Bernard.  (Imagine trying to convince an alien biologist that those two are members of the same species.)  So very quickly, scientists were forced into refining the definition so as to capture the separateness of two different species in such a way that the term could be applied consistently.

What they ultimately landed on was the canonical definition used in just about every biology textbook in the world: "Members of the same species are capable of potentially interbreeding and producing viable and fertile offspring."  (The "fertile" part had to be added because of the famous example of a horse and a donkey being able to produce a viable hybrid -- but that hybrid, the mule, is almost always infertile.)

The problem was, even that wasn't enough to clarify things. Polar bears and grizzly bears, for example, can and do hybridize in the wild, and the offspring (the rather unfortunately-named "pizzly bear") are almost always fertile.  This isn't an aberration.  These kinds of situations are common in the wild.  In fact, in my part of the world, there are two birds that look dramatically different -- the blue-winged warbler and the golden-winged warbler -- but they will happily crossbreed.  When the hybrids were first observed by scientists, they were different enough from both parents that it was thought they were a third separate species, which was called Brewster's warbler.  It was only after long observation that biologists figured out what was going on -- especially given that "Brewster's warblers" are potentially interfertile with either parental species.

In fact, the more you press the definition, the more it falls apart, the more exceptions you find.  Today's taxonomists are usually wary about labeling something a "species" -- or when they do, they're aware that it's potentially an artificial distinction that has no particular technical relevance.  They are much more comfortable talking about genetic overlap and most recent common ancestry, which at least are measurable.

The reason all this comes up is because of a startling discovery brought to my attention by a friend and long-time loyal reader of Skeptophilia.  Researchers in Hungary have produced a hybrid between an American paddlefish and a Russian sturgeon -- two species no one could confuse with each other -- and they appear to be fertile, and normal in every other way.

The more you look at these "sturddlefish," the more shocking they get.  Sturgeon and paddlefish are not only separate species, they're in separate families -- two layers of classification above species.  "I’m still confused," said Prosanta Chakrabarty, ichthyologist at Louisiana State University.  "My jaw is still on the floor.  It’s like if they had a cow and a giraffe make a baby."

He quickly amended that statement -- giraffes and cows have a recent common ancestor only a few million years ago, whereas paddlefish and sturgeons have been separate lineages for 184 million years.  To get anything comparable, Chakrabarty said, you'd have to have something like a human coming out of a platypus egg.

The scientists believe that the reason this happened is because of the relatively slow rate of evolution of both lineages (especially the sturgeons).  Sturgeons now look pretty similar to sturgeons two hundred million years ago, while almost all of the mammalian biodiversity you see around you -- divergence between, say, a raccoon and a squirrel -- happened since the Cretaceous Extinction, 66 million years ago.  But even so, it's pretty remarkable.  To my eye, paddlefish and sturgeon look way more different than lots of pairs of species that can't interbreed, so once again, we're confronted with the fact that the concept of species isn't what we thought it was -- if it has any biological relevance at all.

Atlantic Sturgeon (Acipenser oxyrhynchus)

American Paddlefish (Polyodon spathula)  [Both this and the above image are in the Public Domain]

This brings us back to the unsettling (but exciting) fact that whenever we think we have everything figured out, nature reaches out and astonishes us.  It's why I'll never tire of biology -- to paraphrase Socrates, the more we know, the more we realize how little we know.

But one thing I know for sure is that the biologists really need to come up with better names than "sturddlefish" and "pizzly bear."

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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!]

Grandpa the pig

It bears mention that having a Ph.D. (or other advanced credentials) is no guarantee against being a complete wingnut.  This topic comes up because of a website link sent to me by a regular reader of Skeptophilia that was authored by Eugene McCarthy, Ph.D. in genetics, and author of Handbook of Avian Hybrids of the World.

It starts off reasonably enough; McCarthy describes the fact that, contrary to our perception of species as being little watertight compartments, hybridization (and thus gene flow between species) is rather common.  Not all hybrids are sterile, like the familiar example of the mule; a lot of them are back-fertile to either parental species (an example is the "Brewster's Warbler," which was once thought to be a separate species and is now known to be a hybrid between the Golden-winged and Blue-winged Warblers).

So McCarthy asks an interesting question: are humans a hybrid?  The answer, apparently, is yes; recent studies have shown that most human populations show the genetic signature of three ancestral populations -- modern humans, Neanderthals, and Denisovans.  (Biologists disagree, however, as to whether these three represent different species -- a distinction that, in reality, probably doesn't mean very much.  The concept of species is one of the hardest-to-pin-down terms in all of biological science.)

But, unfortunately, it isn't this intermixing between three proto-hominins that McCarthy is talking about.  He thinks we're a much more interesting hybrid than that.  He gives his evidence first: humans have low fertility, and males produce a great many abnormal sperm (kind of a surprise given our reproductive success -- you have to wonder, if this is true, how there can be seven billion of us).

What?  You want more evidence than that?  Sorry, that's it.  Guys produce lots of abnormal sperm, and allegedly we have low fertility.  So we're hybrids.  That's enough, right?

Of course right.  So now, if we're hybrids, we have to figure out which two species gave rise to humans.  One of them, McCarthy says, was clearly something like a chimp.  But he states, in all apparent seriousness, "Many characteristics that clearly distinguish humans from chimps have been noted by various authorities over the years."  Can't argue with that.  But then he goes right off the edge of the cliff:

One fact, however, suggests the need for an open mind: as it turns out, many features that distinguish humans from chimpanzees also distinguish them from all other primates. Features found in human beings, but not in other primates, cannot be accounted for by hybridization of a primate with some other primate. If hybridization is to explain such features, the cross will have to be between a chimpanzee and a nonprimate — an unusual, distant cross to create an unusual creature.

If this sets alarm bells off, good -- because this would require a fertile hybrid being produced from a mating of animals not just from two different genera, or two different families, but two different orders.  Entirely possible, McCarthy says, despite the fact that there is not a single example -- not one -- of an interordinal hybrid known from nature.  Anywhere.  That includes animals, plants, fungi, and so on.

Nevertheless, that doesn't stop McCarthy:

Looking at a subset of the listed traits [unique features of humans are listed in the sidebar on page two of his website; there are too many to list here], however, it's clear that the other parent in this hypothetical cross that produced the first human would be an intelligent animal with a protrusive, cartilaginous nose, a thick layer of subcutaneous fat, short digits, and a naked skin. It would be terrestrial, not arboreal, and adaptable to a wide range of foods and environments. 

So, let's not dillydally any more; if a chimp is one of our parental species, what's the other?

What is this other animal that has all these traits? The answer is Sus scrofa, the ordinary pig. What are we to think of this fact? If we conclude that pigs did in fact cross with apes to produce the human race, then an avalanche of old ideas must crash to the earth. But, of course, the usual response to any new perspective is "That can't be right, because I don't already believe it." This is the very response that many people had when Darwin first proposed that humans might be descended from apes, an idea that was perceived as ridiculous, or even as subversive and dangerous. And yet, today this exact viewpoint is widely entertained. Its wide acceptance can be attributed primarily to the established fact that humans hold many traits in common with primates. That's what made it convincing... Let us take it as our hypothesis, then, that humans are the product of ancient hybridization between pig and chimpanzee.

So, basically, the logic is, "people laughed at Darwin, and he turned out to be right, so if people laugh at me, I must be right?"

But I don't want to be accused of jumping to conclusions ("That can't be right, because I don't already believe it"), so I took what I think is a critical look at the list of allegedly unique features of humans -- ones that, in McCarthy's view, must have come from our other, non-primate parental species.  And most of them have to do with quantities and sizes -- "sparse" hair, "large amounts" of elastic fiber in the skin, "richly" vascularized dermis, "narrow" eye opening, "heavy" eyelashes, and so on.  Traits involving quantities and sizes are highly responsive to selective pressures, the idea being once you have genes for the production of a feature, it is relatively straightforward to evolve to produce more or less of it.

Of the features he claims are found only in humans and pigs, it appears that in several cases, he is simply wrong.  Take multipyramidal kidneys -- he is correct that only humans have this feature amongst primates, but it is hardly unique in the mammalian world.  Besides humans and pigs, elephants have multipyramidal kidneys, as do bears, rhinoceroses, bison, and "nearly all marine mammals," according to a paper by M. F. Williams (available here).  Williams' contention is that multipyramidal kidneys evolved in animals that lived in coastal or marine environments in order to deal with high levels of salt -- and that each of these lineages evolved it independently, as it represents a unique feature on separate, distantly related branches of the phylogenetic tree (evolutionary biologists call these features "apomorphies").

Then, of course, he has some things on the list of allegedly unique human characteristics that are simply weird.  "Particular about place of defecation?"  (Has he ever owned a cat?)  "Snuggling?"  "Extended male copulation time?"  "Good swimmer?"

I'm sorry, Dr. McCarthy, but I'm calling bullshit on this.

Now, please understand; it's not like I have any particular problem with our having a checkered ancestry.  I'm an evolutionary biologist by training, for cryin' in the sink, I know we're animals.  But the idea that Homo sapiens arose when a chimp had sex with a pig... that stretches credulity too far.

Even if you do have a Ph.D.