A Carboniferous arachnid

This week has been a good one for chelicerate evolution. Here’s another fossil, Douglassarachne acanthopoda, which was creeping around in the forests of Illinois in the late Carboniferous.

Douglassarachne acanthopoda n. gen. n. sp., holotype and only known specimen FMNH PE 91366; for interpretative drawings and scale, see Figure 2. (1) Part, detail of distal femur and more-distal podomeres, showing nature of curved macrospines on lateral edge of distal podomeres, bases of macrospines on dorsal surface of femur; (2) counterpart, detail of posterior opisthosoma showing bilobed structure at base of anal tubercle.

What is it? I don’t know. The authors are unsure. It’s an arachnid, but it could be in the spider lineage or the harvestman lineage, or it could be its own weird thing. It’s spiderish, anyway.

Douglassarachne acanthopoda n. gen. n. sp., reconstruction of the possible appearance of the animal in life.

An Ordovician ancestor to spiders

On this Memorial Day, I’m going to have to have a discussion with my spiders about their distinguished, noble ancestry. It was kind of Nature to publish a study of their many-times-great grand uncles, an ancient euchelicerate named Setapedites abundantis, a common fossil found in Moroccan sediments that are about 478 million years old, which puts it right in the middle of the Great Ordovician Biodiversification Event, a key moment in the evolution of modern taxa.

This is not a spider, though. It belongs in the euchelicerata, the large systematic group that includes spiders, scorpions, ticks, horseshoe crabs, sea scorpions, and other extinct groups. As you might guess from the name, a key feature is the presence of chelicerae, anterior appendages that in spiders carry the venomous fangs. It also has a common feature we see in both spiders and horseshoe crabs, the fusion of the anterior segments to form a prosoma, with posterior segments forming the abdomen or opisthosoma.

While it’s a cool looking little dude, it’s marine and pretty remote from modern chelicerates. From the dorsal side, it looks like an undistinguished little crustacean, of a type that was probably swarming in Ordovician seas.

A, B MGL.102899 and interpretative drawing, articulated specimen in dorsal view. C, D MGL.102828 and interpretative drawing, articulated specimen in dorsal view. E, F MGL. 102872 and interpretative drawing, articulated specimen in dorsal view. Abbreviations: btg, bipartite tergites; mr, median ridge; pl, pleura; pr, prosomal rim; saxn, sub-axial node; sr, sunken region; t1–11, tergites 1–11; t, telson; tk, telson keel. Scale bars, (A–F) 1 mm.

Where it gets interesting is when it’s flipped over, and you get a glimpse of the mass of limbs.

A, B YPM IP 517932c and interpretative drawing (counterpart), articulated specimen in ventral view. C, D YPM IP 517932c and interpretative drawing, chelicerae, and labrum anatomy detail. E, F Close-up of the prosoma of MGL.102934 and interpretative drawing, in dorso-lateral view. G, H Close-up of the prosoma of MGL.102634 and interpretative drawing, in ventral view. I, J Close-up of the prosoma of MGL.102800a under alcohol and polarized lighting, and interpretative drawing, in ventral view. Abbreviations: 1–6, podomeres 1–6 of the exopod; ptp, pretelsonic process; bs, basipodite; bst, brush-like setae; che, chelate podomere; db, doublure; lb, labrum; ss, single setae; st, pair of setae. Chelicerae are highlighted in gray, endopods in blue, exopods in green, opisthosomal appendages in red, and the pretelsonic process in purple. Scale bars, (A, B) 1 mm; (C, D) 100 µm; (E–K) 500 µm.

In front of the jaws proper (labrum, lb) it has a pair of small chelicerae (che). These have since evolved into the massive, sharptoothed chompers you can see my tarantula using to turn a mealworm into macerated mush.

Setapedites wasn’t such a fierce predator. Here’s what it looked like.

Illustration by Elissa Sorojsrisom.

Cute, right? I don’t know why it’s drawn as a swimmer, though — with that anatomy, it looks more like a benthic organism.

The final bit of interesting information is that they mapped out the correspondences in the segmentation of this animal with other, similar fossils and the extant Xiphosurians.

Simplified extended majority rule tree of a Bayesian analysis chronogram of euchelicerate relationships, based on a matrix of 39 taxa and 114 discrete characters, showing the position of Setapedites abundantis within Offacolidae. Lineages extending after the Silurian are indicated with arrowheads. Schematic models of the body organization in Habelia, Setapedites abundantis, Dibasterium, Offacolus, and Xiphosurida illustrate the origin and early evolution of euchelicerate uniramous prosomal appendages and tagmosis. Roman numbers designate somites. Prosoma somites are highlighted in blue, pre-abdomen somites in yellow, abdomen somites in brown, and the possible anal pouch or post-ventral structure (pvs) in purple. Black dorsal lines indicate tergites and cephalotorax. Schematic model of Xiphosurida Offacolus, and Dibasterium from 45, Habelia

Also of note: Setapedites had biramous appendages, a feature that is mostly kind of lost in modern arthropods — the outer branch got adapted into gills and lungs and even wings.

I can’t help but notice that domestication and artificial selection turns wolves in little yapping Pomeranians, but natural selection turns shrimp into tarantulas.

Everyone is talking about Homo naledi now

Since I mentioned the fun time I had in class getting students to wrestle with the claims of Homo naledi‘s superpowers, it was a nice coincidence that Gutsick Gibbon made a video about the very same conversation, right down to the same papers I gave my students to read.

I did have a moment of concern — am I pushing graduate level content on first year undergrads? Nah, they handled it fine and even came up with some of the same points mentioned in the video.

Burying the dead

If you have a subscription to Netflix, you might want to watch Unknown: Cave of Bones, about the discovery of Homo naledi in the Rising Star cave system. It’s spectacular.

On the other hand, if you’re claustrophobic, you might want to skip it. I’m not particularly, but I watched the video of those women wriggling their way down a narrow crack to reach the Dinaledi Chamber gave me a rising sense of panic. There’s no way I could put myself in that position without having a screaming heebie-jeebie fit.

If you can get past that, though, it’s worth it to watch the adventure of science.

Dear god I hate this

This is terrible. All that graphics arts skill gone to waste as someone tries to illustrated something they don’t understand.

Evolution as a linear path towards a particular, narcissistic goal (of course, it’s all about us!) The implications of a teleological purpose. The self-centeredness. The trimming of diversity. Notice how everyone under genus Homo is stereotypically male — all the prior stages may have been masculine in the artist’s mind, too, but it’s hard to tell. This is going to fuel so many continued misconceptions.

At least the conclusion mentions that “Humans are still evolving,” but they should have shut up with that, rather than making predictions. Why should we all converge on a “‘Great Averaging’ where continuous, international mixing will create an average of all current physical differences”? Will drift and mutation stop? Will all population structures break down? Do chance and varied environmental pressures disappear, to favor one pattern of humans being “taller, more lightly built, and less aggressive with smaller brains”? Well, maybe the “smaller brains” is a reasonable extrapolation from our current experience.

I see the creator has “two Master of Science degrees (one in Geochemistry/Astrobiology, another in Biomedical Communications).” I can tell evolutionary biology wasn’t one of them.

It takes multiple mechanisms to drive evolution

I went through high school thinking that evolution was pretty neat. I read all the popular treatments — those Time/Life books, National Geographic, science magazines, etc. — and they made it sound like Darwin had got everything right in 1859. I could go along with that.

But then, in 1975, I went off to college and had access to a university library and started reading the real hardcore stuff. I was just mainlining PNAS & Science & Nature & Evolution & the Journal of Embryology and Experimental Morphology and soaking in all this exciting stuff, and that’s when I discovered the concept of genetic drift (specifically, thanks to Lewontin). My mind was blown. Suddenly, questions I had never even thought to ask were answered with clarity. Certainly nothing in biology makes sense except in the light of evolution, but likewise nothing in evolution makes sense except in the light of neutral genetic drift.

I guess Holly Dunsworth had the same revelation, and also noticed the sad gap in public education.

In the USA, K-12 evolution education standards are missing GENETIC DRIFT as well as the word NEUTRAL.

(see here: https://www.nextgenscience.org/topic-arrangement/hsnatural-selection-and-evolution)

I don’t know how to understand life’s perpetual change without those fundamentals.

If students know about meiosis (which *is* in the standards), then they know about genetic drift. It’s just a matter of linking meiosis to evolution.

Genetic drift is a very simple concept and makes natural selection make a whole lot more sense!

I shout about this on Twitter and I get “just be happy anyone teaches evolution in K-12 at all” and “teachers don’t know about drift” in response. But evolution without genetic drift is not evolution, and if teachers knew about drift, then they’d be more comfortable teaching evolution. I guarantee it!

Without drift, it’s too easy to just replace God with natural selection. And that habit of narrating evolution by giving agency to natural selection is super-duper weird for non-believers let alone for believers! (This a no hatred of religion, faith, or creationism zone.)

Genetic drift paves the way for thinking about and then narrating evolution as the constant change that life/nature/biology is. Everyone gets that time is just constant change and they will get that nature/biology/life is too. They embody it themselves, being different from their parents.

Plus, continuing to teach evolution as only being natural selection (which is what the standards are doing), is also dangerous. That selection-obsessed mindset is tied to racism, sexism, essentialism… all the stuff that we have to remove from our species’ shared origin story. Darwin only had selection (not drift, etc) to work with and look what his imagination did with evolution: racism, sexism, essentialism

Oh wow, yes. I’ve got to stand up and cheer at that. Also specifically the bit about meiosis, because I’m teaching genetics right now and am constantly emphasizing the role of chance in inheritance. We’re built on a platform of coin flips and die rolls and long shots in heredity, yet somehow in our public education, that all vanishes and is replaced with something close to fixed destiny (in higher ed, that isn’t true, fortunately. Usually.)

Also, drift nicely bridges all those gaps in comprehension the creationists love to inflict on us.

Fire, 250,000 years ago

The experts are now explaining that Homo naledi almost certainly used fire to explore caves & cook food. This isn’t huge surprise, because how else could they have maneuvered through the pitch black darkness of an elaborate cave system?

Last August, Berger climbed down a narrow shaft and examined two underground chambers where H. naledi fossils had been found. He noticed stalactites and thin rock sheets that had partly grown over older ceiling surfaces. Those surfaces displayed blackened, burned areas and were also dotted by what appeared to be soot particles, Berger said.

Meanwhile, expedition codirector and Wits paleoanthropologist Keneiloe Molopyane led excavations of a nearby cave chamber. There, the researchers uncovered two small fireplaces containing charred bits of wood, and burned bones of antelopes and other animals. Remains of a fireplace and nearby burned animal bones were then discovered in a more remote cave chamber where H. naledi fossils have been found.

This is where the creationists are going to have to get creative in their excuses, because many of them (including Answers in Genesis) have declared that H. naledi was just an ape. Joel Duff explains the conundrum they face.

But don’t worry, they’ll just invent some improbable impossible ridiculous bullshit to justify whatever conclusion they want.

Oh no, it’s a meme

Herbert Spencer would be so proud.

One quibble: evolution by natural selection isn’t the fundamental concept, it’s evolution, change, by multiple mechanisms.

The red button comic is also relevant.

Yeah, because it’s virtually impossible to teach a creationist anything.

A nice summary of human evolution

It’s a listicle, of course, but it’s written by an archaeologist, which is good, but she was the recipient of a Templeton grant, which triggers my skepticism. But she makes 6 points about our history:

Species such as Homo Longi have only been identified as recently as 2018. There are now 21 known species of human.

And 20 of them went extinct. We’re part of a dying tree.

Hybrid species of human, once seen by experts as science fiction, may have played a key role in our evolution. Evidence of the importance of hybrids comes from genetics. The trail is not only in the DNA of our own species (which often includes important genes inherited from Neanderthals) but also skeletons of hybrids.

Our sexy ancestors were mating with everything that looked vaguely human. Keep that in mind when the dudebros are whining about how Western women aren’t meeting their high standards of beauty. They’d fuck a chimp if one offered the opportunity.

However, many of the changes in our human evolutionary lineage maybe the result of chance.

For example, where isolated populations have a characteristic, such as some aspect of their appearance, which doesn’t make much difference to their survival and this form continues to change in descendants. Features of Neanderthals’ faces (such as their pronounced brows) or body (including large rib cages) might have resulted simply from genetic drift.

Oh, dear, the evolutionary psychologist won’t like that. Too bad, it’s true.

The origins of our own species coincided with major shifts in climate as we became more distinct from other species at these points in time. All other species of human seem to have died out as a result of climate change.

…and we’re next, at the rate we’re going.

The trail of human compassion extends back one and a half million years ago. Scientist have traced medical knowledge to at least the time of the Neanderthals.

Altruism has many important survival benefits. It enabled older community members to pass on important knowledge. And medical care kept skilled hunters alive.

We should have listened more closely to Kropotkin, rather than the imperialist colonizers who shaped the early perspectives on evolutionary theory. He was on to something (he also considered climate to be a critical evolutionary force.)

Evolution made us more emotionally exposed than we like to imagine. Like domestic dogs, with whom we share many genetic adaptations, such as greater tolerance for outsiders, and sensitivity to social cues, human hypersociability has come with a price: emotional vulnerabilities.

Counterpoint: Republicans and Tories. Human psychopaths seem to have a decided advantage in the acquisition of power.