Cool data!

Nick Matzke has
compiled all the data on hominin cranial capacities into a single chart:


I think I can see a pattern there, can you? He also has data on body size and brain size over there, take a gander at it. It looks like a simple and obvious example of evolutionary change in our lineage, I think.

Alas, it only shows specimens older than 10,000 years. I’m sure that right around 6,000 years ago, there was a sudden, dramatic change as the deity injected a soul into those crania.

You must be kidding, Mr Unwin

Here’s another review of Dawkins’ The God Delusion(amzn/b&n/abe/pwll). It’s unbelievable, as if the critic hadn’t actually read the book. Here’s the hed/dek:

Dawkins needs to show some doubt
Scientists work in a field full of uncertainties. So how can some be so sure God doesn’t exist? asks Stephen Unwin

Uh, what? Two things immediately come to mind: certainty isn’t a claim Dawkins makes anywhere, and…Stephen Unwin???!? Unwin is a remarkably silly man, as anyone who has read his book, The Probability of God will know. Unwin goes on with some very strange inferences.

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Hox complexity

Here’s a prediction for you: the image below is going to appear in a lot of textbooks in the near future.

(click for larger image)

Confocal image of septuple in situ hybridization exhibiting the spatial expression of Hox gene transcripts in a developing Drosophila embryo. Stage 11 germband extended embryo (anterior to the left) is stained for labial (lab), Deformed (Dfd), Sex combs reduced (Scr), Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), Abdominal-B (Abd-B). Their orthologous relationships to vertebrate Hox homology groups are indicated below each gene.

That’s a technical tour-de-force: it’s a confocal image of a Drosophila embryo, stained with 7 fluorescent probes against different Hox genes. You can clearly see how they are laid out in order from the head end (at the left) to the tail end (which extends to the right, and then jackknifes over the top). Canonically, that order of expression along the body axis corresponds to the order of the genes in a cluster on the DNA, a property called colinearity. I’ve recently described work that shows that, in some organisms, colinearity breaks down. That colinearity seems to be a consequence of a primitive pattern of regulation that coupled the timing of development to the spatial arrangements of the tissues, and many organisms have evolved more sophisticated control of these patterning genes, making the old regulators obsolete…and allowing the clusters to break up without extreme consequences to the animal. A new review in Science by Lemons and McGinnis that surveys Hox gene clusters in different lineages shows that the control of the Hox genes is much, much more complicated than previously thought.

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Flock of Dodos

Earlier this week, I had a chance to talk with Randy Olson about this business of communication good science to the public. I’ve had some disagreements with his strategies before; I think we resolved them a bit. What I had interpreted as a call to dumb down science to get it to the people is really a request that we develop clear narratives, good stories, and sharp, comprehensible slogans backing evolution and science teaching. I agree completely. We are experts at efficient discourse within the community of science, but when it comes to talking to middle America, we suck. There is a good reason for that — we get all of our training in how to talk to other people who have all of our training, but not in how to educate people who don’t have the same background — but that’s no excuse. It’s something we have to change.

Randy was generous and let me have a copy of his movie, Flock of Dodos, and I finally found time to sit down and watch it this evening. It’s excellent and the overall message was one with which I agree, and I hope more scientists get a chance to see it—it accomplishes its mission of shaking us up and pointing out our flaws, and we need that. However, it doesn’t quite satisfy the criticisms I had in mind before I saw it, and there was something that bugged me throughout.

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Cellular responses to alcohol

Forgive me, but I’ll inflict a few more zebrafish videos on you. YouTube makes this fun and easy, and I’m going to be giving my students instruction in video micrography next week, so it’s good practice.

This is a more detailed look at what’s going on in the embryo. Using a 40x objective, we zoom in on a patch of cells near the surface of a 4-hour-old embryo—this is a generic tissue called the blastoderm. We just record activity with an 1800-fold time compression for a few hours to see what the cells are doing. The movie below displays typical, baseline activity: the cells are jostling about, you’ll see an occasional mitosis, and sometimes you’ll see a cell vanish out of focus as it moves deeper into the embryo, and sometimes you’ll suddenly see a new cell squirm to the surface. It’s all just a happy, dynamic place with lots of random motion; these can be mesmerizing to watch.

These blastoderm sheets are a kind of cellular testbed for quick assays of the effects of teratogens on embryonic tissues. We just wash the embryo with whatever substance we’re interested in testing, and see if and how the cells react.

Alcohol is a dramatic example. Here’s a blastoderm sheet under stress as it is exposed to 3% ethanol.

Some obvious changes are going on. One is that the surfaces of the individual cells are seething—they are bubbling out and sucking back in little balloons of membrane, a process called blebbing. This is a very typical response to any kind of stress. Apparently, mitosis is another kind of stress: we can reduce the concentration of alcohol so that the cells look normal, except that as they’re about to divide they go into a flurry of blebbing that persists until division is complete.

We had another puzzle to solve. Sometimes, as we were looking at our low magnification recordings of embryos, we’d see the whole blastula or gastrula shudder. They don’t have muscles yet! We didn’t know what was causing pulses of contractile activity to sweep across the whole animal at such a relatively undifferentiated stage.

These movies show what was going on. They’re a real pain to keep in focus, because in addition to the fine blebbing activity in individual cells, the whole surface occasionally dimples and changes shape. What’s happening? Cells are dying somewhat randomly, some on the surface, some deeper in the embryo. Deep cells that die seem to be actively evicted from interior; sometimes the surface will buckle inward (with the image going out of focus), and when it bounces back up, it ejects a load of cellular debris out into the external medium. There’s a particularly dramatic example at the end of this movie, where everything in the lower half goes massively out of focus, and when it bounces back, it carries a large dead cell that sits there briefly, then abruptly pops and disappears.

If you look at that earlier lower resolution movie of ethanol effects, you might notice odd rough blobs on the surface of the embryo, and we think what that is is the extruded debris of deep cells killed by alcohol exposure, thrown up out of the interior to prevent them from interfering with normal development. This is actually a rather cool cellular mechanism that helps embryos survive random glitches in the process of building these massive pools of cells as it grows—it’s a kind of tissue-level garbage disposal service.