Category Archives: evolution

The Latest Extinction Event Began Thousands of Years Ago

Some researchers in Denmark have released a map of what global mammalian megafauna distribution would be like if humans hadn’t existed. Spoiler Alert: There would be a lot more big mammals everywhere. I don’t think this is a surprise to anyone who has paid any attention to species that have gone extinct but which human ancestors dealt with (I think the problem is that everyone conflates “extinct” into one period, from the dinosaurs to the dodo bird), but this is the first time that I know of that it has been quantified and displayed in an easy-to-understand graphic. Here’s the map:
largemammalsdistribution
I’ve often argued that the reason there are still so many large mammals in Africa is because they evolved with humans, and everywhere else they evolved without humans – invasive species tend to mess things up. The authors agree with me, and go further to say that many of our ideas about diversity are wrong. For example:

Today, there is a particularly large number of mammal species in mountainous areas. This is often interpreted as a consequence of environmental variation, where different species have evolved in deep valleys and high mountains. According to the new study, however, this trend is much weaker when the natural patterns are considered.

“The current high level of biodiversity in mountainous areas is partly due to the fact that the mountains have acted as a refuge for species in relation to hunting and habitat destruction, rather than being a purely natural pattern. An example in Europe is the brown bear, which now virtually only live in mountainous regions because it has been exterminated from the more accessible and most often more densely populated lowland areas,” explains Soren Faurby.

I found the press release through IFLS, and Josh Davis writes at the bottom:

The study does, however, presume that the changing climate at the end of the Pleistocene was not sufficient to kill the large mammals off on its own, and that it was man’s influence that delivered the death blow. This area of research is hotly debated and contested, with arguments flying back and forth as to the real reason the world’s large mammals died out. It’s generally thought likely to be a combination of climate change and hunting, but it’s impossible to say whether or not all species of mammal would have been able to adapt sufficiently to a changing environment and survive to present.

I can’t help but think arguments against human causes are simply trying to avoid guilt. Let’s look at Australia: The continent had a vast diversity of mammalian megafauna for tens of millions of years, megafauna that survived ice ages and warm times, and – most relevant – several glaciation events within the current ice age. Along come humans, and during the very next glaciation event that happens many, many species become extinct. The significant difference was the introduction of humans. Sure, it’s “impossible” to say whether all species of mammals would have been able to adapt, but it’s a pretty safe bet that most of them would have, because they had before.

The Multicellularity of Alien Life Forms

The latest news out of biology (well, it’s been building for quite a long time) is that the tree of life is very different from how we thought it was. The bit that got me excited?

In the new vision — based on increasingly sophisticated genetic analyses — people and other animals are closer cousins to single-celled choanoflagellates than to other multi­cellular organisms. Giant kelp that grow as wavering undersea forests off the California coast are closer relatives to single­-celled plankton called diatoms than to multicelled red seaweeds or plants.

This is because it obliterates the notion that multicelluarity evolved only once. In fact, Wikipedia claims “complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and land plants. It evolved repeatedly for Chloroplastida (green algae and land plants), once or twice for animals, once for brown algae, three times in the fungi (chytrids, ascomycetes and basidiomycetes) and perhaps several times for slime molds, and red algae.” This is cool because it removes one of the bottlenecks for intelligent life: The chances of life spontaneously evolving are vanishingly small (once that we know of), and the chance of eukaryotic life developing is also vanishingly small (once that we know of). However, where in the past it was thought* that multicellularity evolved only once, now it seems a common, and therefore inevitable, occurrence.

Which means, if we do find extraterrestrial life, it will almost certainly be multicellular in some way.

*At least by me, and it seemed to be the general thought when I went through uni 20 years ago. I don’t know how long scientists dedicated to the field have been aware of this.

For more alien life science, see Life In Cryogenic Conditions.

The Free Market in the Bacterial World

Bacteria can exchange vital nutrients with other bacteria, effectively distributing metabolic functions within the microbial communities. This is one of things that was supposed to differentiate unicellular life from multicellular life…

As reported in io9:

Just over a year ago, the lab of Christian Kost, based out of the Max Planck Institute for Chemical Ecology in Jena, Germany, showed that two strains of bacteria, genetically engineered to lack one nutrient and overproduce another, could support one another in the same flask if their deficiencies and surpluses were complementary. On their own, either strain would not survive. (The nutrients were the two amino acids, histidine and tryptophan).

These bacteria grew 20 percent faster than strains that didn’t require the exchange of nutrients, indicating a benefit to specialisation (of the kind that would be familiar to any beginner economics student). Anyway, the really interesting news here is that the nutrients aren’t excreted into the media and absorbed by other bacteria; a bacterium that lacks one of the nutrients will actively seek out other bacteria that have the nutrient and then hook up a tube to them to exchange nutrients. This could be mutualistic or parasitic, but either way it’s pretty high-level behaviour for a single celled organism.

Of course, it has been known that bacteria exchange DNA and RNA through this method for quite a few years and its pretty clear this is happening in the wild as well as in the lab – and between bacteria of different species.

This has significant implications for the way we think bacteria evolve (although I don’t think many microbiologists will find this result particularly revolutionary, more another piece in the puzzle). It’s not just that bacteria don’t rely on mutations to adapt, they can pass genes between each other, now it turns out they might outsource some metabolic pathways. When a particular compound becomes scarce and they can’t synthesise it, they just seek out other bacteria that can.

There’s still some questions, of course, including how the bacterium can tell which other bacterium to attach to, and whether the other has to be “willing”. However, the idea of bacteria as individual organisms swimming alone in their environment in competition with everything they come across should be dead in your head by now.

(image source: Electron microscopy image of E. coli using nanotubes to survive off of a continuous trade of amino acids with other bacteria. Martin Westermann, University Hospital of the University of Jena.)

Is There an Evolutionary Explanation for PMS?

One of the bugbears of our times is PMS, a physical condition that’s used as everything from a punchline to an idiotic reason why women shouldn’t be in charge of something important. Why does PMS exist? Is there any advantage to PMS, one that could have been selected for over the course of our evolutionary history?

A recent paper hypothesizes that the evolutionary advantage of PMS is that it “increased the chance that infertile pair bonds would dissolve, thus improving the reproductive outcomes of women in such partnerships”. Michael Gillings bases this idea around the fact that women don’t get PMS when they’re pregnant, so PMS is a sign of an infertile pairing.

There are three legs of his argument detailed in the paper published in Evolutionary Applications: Genetic variation affects the severity of PMS; PMS is directed preferentially at current partners; and PMS increases the likelihood of changing partners.

He cites studies that show that genetic variation affects the severity of symptoms of PMS, and that these symptoms are hereditary. This variation is normally put down to differing sensibility to the hormones, since “the absolute levels of progesterone, oestrogen and testosterone do not differ between individuals at high or low risk of PMS”. Since this genetic variation is heritable, it could be subject to natural selection.

Gillings cites a number of studies showing that severe PMS has a negative impact on marital relations, which is no surprise, but only one study which shows that it affects family life greater than social, school and occupational life. Specifically, “Functional impairment tended to be highest at home, followed by social, school, and occupational situations.” (source) This could be explained by the tolerance for functional impairment going from highest to lowest in those situations. At work you do your job, even if you’re in pain – at home you leave the dishes for later and just order pizza for dinner, because who’s going to say otherwise.

However, the crux of his argument is that PMS encourages a change of sexual partners, and for this he has little support. Most of his references in this section are of tendency towards risk, whether or not women want to have sex at this point in their cycle, and whether visible signs of menstruation would encourage other males to attempt to mate with a woman (this connection is not only a stretch, but irrelevant to PMS). This section is full of segues and studies only loosely related to what he’s trying to show.

It’s not a strong hypothesis, and ScienceNews interviewed a number of researchers who were generally unimpressed. A very strong point was made by Jane Ussher, a women’s health psychologist at the University of Western Sydney in Australia, who said: “Women are fertile before the premenstrual phase of the cycle…so any function of PMS in terms of repelling males would have no impact on fertility.” She goes on to add that PMS directed at partners is probably a symptom of other issues the woman is unhappy about.

For me, the most compelling argument was made by Mark Elgar, who studies evolutionary biology at the University of Melbourne in Australia. He pointed out that there’s no association between PMS symptoms and how many children a woman has. “In other words,” he explains, “there doesn’t seem to be a very compelling reason to construct an evolutionary explanation of PMS in the first place.”

PregnancyBar

Speculative hypotheses are integral to the advancement of scientific knowledge, but this seems to go off on an unnecessary side tangent. Hormones and their receptors play important roles in a variety of traits that are under selective pressure, while menstruation is a vitally important aspect of human reproduction. Menstruation can be painful, and pain makes people irritable; menstruation also requires a change in the levels of hormones, which naturally changes their other functions. PMS is just a byproduct of these biological aspects. Gillings – who lectures on human biology and the science of sex – seems to be seeking an evolutionary explanation for a trait where none is required. There is an unfortunately common idea that every single trait must have an evolutionary explanation, and must have conferred some sort of benefit that was selected for. In 1979 Stephen J Gould and Richard Lewontin published a paper (pdf) arguing against this position (which I’ve remembered because it has the fantastic title of: The Spandrels of San Marco and the Panglossian Paradigm). They made the point that many traits are simply byproducts of other things that have been selected for, and convey no inherent advantage themselves. This is probably what PMS, since it doesn’t seem to affect the reproductive capabilities of women.

Also (getting input from someone who actually menstruates), my wife told me that PMS is a largely modern and western phenomenon, and is made worse by low levels of exercise and the use of hygienic products made from synthetic materials.