Quick takes

Today I'd like to look at three topics that came up for one reason or another in the last few days.

First, have you ever thought about catnip?  It's peculiar stuff.  A member of the mint family, Nepeta cataria (no, I didn't make up the species name) is a rather un-showy plant with gray-green stems and off-white flowers.  Its strangest characteristic, as I'm sure you know, is that it produces what amounts to kitty drugs.  The chemical nepetalactone is produced in significant quantities by the plant, and is responsible not only for its musky smell but for its apparent psychedelic effects on cats.  Cats who are susceptible to it -- and some, I understand, are not, although every cat I owned was a total catnip junkie -- purr, roll around in it, become playful and frisky, and their eyes dilate.

[Image licensed under the Creative Commons AlishaLH, Bee on Catnip Flowers, CC BY 4.0]

Clearly this is an accident; it's hard to imagine a plausible scenario in which the trait of producing this chemical evolved for the express reason of making cats flail around.  There's a presumption that it has a repellent effect on insects, which is certainly true about a lot of the aromatic substances produced by plants, but that's unsubstantiated.

The reason this comes up is that scientists at the Max Planck Institute of Chemical Ecology have decoded the genome of catnip, and they found that the nepetalactone gene apparently evolved more than once.  There are inactive "pseudogenes," stretches of DNA whose function has been lost over time to mutations, nearly identical to the current (functional) nepetalactone gene.  So evidently the gene evolved a second time, probably from another gene for producing chemicals of the class nepetalactone belongs to (the iridioids, which sounds like an alien species on Doctor Who but isn't), and then was advantageous enough that it kind of went into overdrive in catnip.

Apparently whatever its function is, it's important for more than getting kitties high.

So we're not the only species that has a strange psychological reaction to various naturally-produced chemicals in plants.  It's a good thing, though, that there's no such thing as (for example) moosenip, because the idea of a bunch of moose frolicking about and rolling around in your front yard is a little terrifying.


The next story comes from some research released last week by NASA's Transiting Exoplanet Survey Satellite (TESS), which made an interesting discovery about a rather odd kind of star.  Called Delta Scuti stars (after the first star identified in this class), they are a pulsating variable that are more common than astronomers thought at first -- there's good evidence that the bright stars Altair and Denebola, in the constellations Aquila and Leo respectively, are also in this class.

What makes these stars so odd is not their fluctuations -- periodic variable stars are actually rather common -- but that some parts of the star move outward and dim, and other parts pull inward and brighten, at the same time.  The whole star, therefore, seems to ripple.  The astrophysicists believe this is because as parts of the star heat up, more of the helium in the outer shell becomes ionized; ionized helium is more opaque to light, so it blocks light trying to escape from the core and balloons outward.  Once it cools, it falls back inward, and since this happens at different times in different places on the surface of the star, there are pulsations in not only the overall brightness of the star, but which parts of the star are bright and which are dim.

A paper in Nature last week showed that their behavior may not be as chaotic as it seemed at first.  Data from TESS has shown that these surface fluctuations have their own kind of periodicity -- some, for example, seem to contract and expand one hemisphere at a time, not at random places on the star.

"Delta Scuti stars have been frustrating targets because of their complicated oscillations, so this is a very exciting discovery," said Sarbani Basu, a professor of astronomy at Yale, who studies asteroseismology but was not involved in the study, in an interview in Science Daily.  "Being able to find simple patterns and identify the modes of oscillation is game changing.  Since this subset of stars allows normal seismic analyses, we will finally be able to characterize them properly."

Last, I was contacted by a reader regarding last week's post about the presence of iridescence in the fossilized feathers of ancient birds, with a question as to whether this discovery might shed any light on the presence of tetrachromacy in birds.  You probably know that (most) humans are trichromats -- we have three different kinds of color-detecting cones in the retina of the eye, sensitive to blue, green, and red wavelengths.  The combination of these three gives us our perception of color.

Mammals, apparently, descend from animals that were tetrachromats -- they had four different cones, and presumably, a more highly refined sense of color detection than humans have.  But in most mammals, such as dogs, there were mutations that knocked out two of the genes responsible -- similar to the loss of the catnip gene described earlier -- leaving behind two functioning cone types, and poorer color discrimination.

Some humans -- almost all are female -- have a fourth working cone, and are true tetrachromats.  This is undoubtedly why when my wife and I are going out, a common comment from her is, "Seriously?  You think that shirt matches those pants?"  Saying my aesthetic sense, especially when it has to do with sartorial matters, is poorly developed is kind of a massive understatement.

Birds -- at least the species that have been tested -- seem to all be tetrachromats, which may be why so much of their display behavior has to do with flashing bright colors around.  The presence of feather iridescence in birds from 52 million years ago may be an indication that they've been able to do this for a very long time.

It must be said, however, that the record holder for number of different kinds of color-sensitive photoreceptor is the mantis shrimp, which (depending on species) has between twelve and sixteen independent kinds of cones.  You have to wonder what the world looks like to them, don't you?

So that's three quick takes from the world of science.  And thanks to the reader who suggested a post on tetrachromacy -- it's a fascinating subject, well worth a look.  So keep those cards and letters comin'.

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This week's Skeptophilia book of the week is six years old, but more important today than it was when it was written; Richard Alley's The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future.  Alley tackles the subject of proxy records -- indirect ways we can understand things we weren't around to see, such as the climate thousands of years ago.

The one he focuses on is the characteristics of glacial ice, deposited as snow one winter at a time, leaving behind layers much like the rings in tree trunks.  The chemistry of the ice gives us a clear picture of the global average temperature; the presence (or absence) of contaminants like pollen, windblown dust, volcanic ash, and so on tell us what else might have contributed to the climate at the time.  From that, we can develop a remarkably consistent picture of what the Earth was like, year by year, for the past ten thousand years.

What it tells us as well, though, is a little terrifying; that the climate is not immune to sudden changes.  In recent memory things have been relatively benevolent, at least on a planet-wide view, but that hasn't always been the case.  And the effect of our frantic burning of fossil fuels is leading us toward a climate precipice that there may be no way to turn back from.

The Two-Mile Time Machine should be mandatory reading for the people who are setting our climate policy -- but because that's probably a forlorn hope, it should be mandatory reading for voters.  Because the long-term habitability of the planet is what is at stake here, and we cannot afford to make a mistake.

As Richard Branson put it, "There is no Planet B."

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

Bugs for dinner

Every once in a while I'll run across something that is such an elegant support for the theory of evolution that I can't help but wonder how you'd explain it otherwise.

Of course, the evidence for evolution is overwhelming anyhow, and the people who disbelieve it are evaluating the world through a different lens.  But still, I'd like to know how a strict creationist would respond to a beautiful piece of research out of the University of California - Berkeley that was just published last week.

Entitled "Chitinase Genes (CHIAs) Provide Genomic Footprints of a Post-Cretaceous Dietary Radiation in Placental Mammals," the paper, by Christopher A. Emerling, Frédéric Delsuc, and
Michael W. Nachman, at first seems as if it would be of interest only to someone who's keen on population genetics or paleontology.  The authors write:

The end-Cretaceous extinction led to a massive faunal turnover, with placental mammals radiating in the wake of nonavian dinosaurs.  Fossils indicate that Cretaceous stem placentals were generally insectivorous, whereas their earliest Cenozoic descendants occupied a variety of dietary niches.  It is hypothesized that this dietary radiation resulted from the opening of niche space, following the extinction of dinosaurian carnivores and herbivores.  We provide the first genomic evidence for the occurrence and timing of this dietary radiation in placental mammals.  By comparing the genomes of 107 placental mammals, we robustly infer that chitinase genes (CHIAs), encoding enzymes capable of digesting insect exoskeletal chitin, were present as five functional copies in the ancestor of all placental mammals, and the number of functional CHIAs in the genomes of extant species positively correlates with the percentage of invertebrates in their diets.  The diverse repertoire of CHIAs in early placental mammals corroborates fossil evidence of insectivory in Cretaceous eutherians, with descendant lineages repeatedly losing CHIAs beginning at the Cretaceous/Paleogene (K/Pg) boundary as they radiated into noninsectivorous niches.  Furthermore, the timing of gene loss suggests that interordinal diversification of placental mammals in the Cretaceous predates the dietary radiation in the early Cenozoic, helping to reconcile a long-standing debate between molecular timetrees and the fossil record.  Our results demonstrate that placental mammal genomes, including humans, retain a molecular record of the post-K/Pg placental adaptive radiation in the form of numerous chitinase pseudogenes.

Put more simply, the authors have discovered a fascinating correspondence:

• Unsurprisingly, insectivorous species have genes to break down chitin, which makes up insect exoskeletons.  If an insectivorous species suffered a mutation in a chitinase gene, it would be likely to die of malnutrition, making those mutations unlikely to survive in the genome.
• If these animals switched diets, then a mutation in a chitinase gene wouldn't be harmful.  So any mutations that occurred wouldn't kill the animal in which they occurred, and they would be maintained in the population.
• Non-insectivorous mammals -- including ourselves -- still have these remnant malfunctioning genes, some of them tens of millions of years old, leftovers in our genomes from our distant, insect-eating ancestors.
• Most fascinatingly, different non-insectivorous lineages -- such as (for example) ourselves and dogs -- both have broken chitinase genes, but the genes aren't broken in the same way.  The mutation that occurred in the lineage that led to Order Carnivora is different from that in the lineage that led to Order Primata.  But within an order, the chitinase "pseudogenes" are very similar.

I'd like a creationist to tell me why, if we did not evolve, we still have broken copies of a gene that would only be useful to a species that fed exclusively on insects.

A cottontop tamarin eating an insect [Image licensed under the Creative Commons Mickey Samuni-Blank, Cottontop tamarin, CC BY-SA 3.0]

Study co-author Christopher Emerling certainly appreciates the import of this discovery:

In essence, we are looking at genomes and they are telling the same story as the fossils: that we think these animals were insectivorous and then dinosaurs went extinct.  After the demise of these large carnivorous and herbivorous reptiles, mammals started changing their diets... 

One of the coolest things is, if you look at humans, at Fido your dog, Whiskers your cat, your horse, your cow; pick any animal, generally speaking, they have remnants in their genomes of a time when mammals were small, probably insectivorous and running around when dinosaurs were still roaming Earth.  It is a signature in your genome that says, once upon a time you were not the dominant group of organisms on Earth. By looking at our genomes, we are looking at this ancestral past and a lifestyle that we don’t even live with anymore.

I'm right there with Emerling in thinking this is amazingly cool.  That's what science is about, you know?  Elucidating some small part of the universe, and in the process, leaving us awestruck.  Myself, I'm just as glad we don't eat insects any more.  Grasshoppers simply don't appeal the way a rare t-bone steak does.  But the idea that we can show that my far distant ancestors, seventy million years ago, dined on bugs -- that is pretty freakin' cool.

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This week's recommended book is an obscure little tome that I first ran into in college.  It's about a scientific hoax -- some chemists who claimed to have discovered what they called "polywater," a polymerized form of water that was highly viscous and stayed liquid from -70 F to 500 F or above.  The book is a fascinating, and often funny, account of an incident that combines confirmation bias with wishful thinking with willful misrepresentation of the evidence.  Anyone who's interested in the history of science or simply in how easy it is to fool the overeager -- you should put Polywater by Felix Franks on your reading list.