Ancient rules for Bilaterian development

Posted 30 July 2006 by PZ Myers

Assuming that none of my readers are perfectly spherical, you all possess notable asymmetries—your top half is different from your bottom half, and your front or ventral half is different from you back or dorsal half. You left and right halves are probably superficially somewhat similar, but internally your organs are arranged in lopsided ways. Even so, the asymmetries are relatively specific: you aren't quite like that Volvox to the right, a ball of cells with specializations scattered randomly within. People predictably have heads on top, eyes in front, arms and legs in useful locations. This is a key feature of development, one so familiar that we take it for granted.

I'd go so far as to suggest that one of the most important events in our evolutionary history was the basic one of taking a symmetrical ball of cells and imposing on it a coordinate system, creating positional information that allowed cells to have specific identities in particular places in the embryo. When the first multicellular colony of identical cells set aside a particular patch of cells to carry out a particular function, say putting one small subset in charge of reproduction, that asymmetry became an anchor point for establishing polarity. If cells could then determine how far away they were from that primitive gonad, evolution could start shaping function by position—maybe cells far away from the gonad could be dedicated to feeding, cells in between to transport, etc., and a specialized multicellular organism could emerge. Those patterns are determined by interactions between genes, and we can try to unravel the evolutionary history of asymmetry with comparative studies of regulatory molecules in early development.

Continue reading "Ancient rules for Bilaterian development" (on Pharyngula)

8 Comments

Bruce Thompson GQ · 30 July 2006

Different lineages specifying cell fates sounds like evolutions first attempt at moving the goal posts. Deep in the past before any thought of ID, blind genetics through the process of random mutation was shifting body plans around. While many organisms settled on a bilateral plan others chose alternate routes. This sneaky attempt at moving the goalposts, presenting biologists with more than one body plan is certainly unfair. It increases the work load dramatically, not only deciphering the mechanisms underlying specific body plans but also all those related genes and various expression patterns.

For the non mechanistic ID, design detection should concentrate on the simplest body plan. Successfully demonstrating design using the simplest body plan will, by default, suggest more complex body plans are also designed.

Delta Pi Gamma (Scientia et Fermentum)

Jack Krebs · 30 July 2006

Very interesting. I've just read part of Sean Carrol's "Endless Forms Most Beautiful," so I knew enough to follow your post.

On the other end of the spectrum, while speaking in Garden City, Kansas last Friday, a YEC in the audience hit me up with the "Haeckel's embryo's" issue. I tried to explain to the audience a bit about how studies about embryology and evolution had advanced so far beyond that simplistic stage that in terms of actual science, the issue of Haeckel's embryo's is nothing but a bit of history: real embryology is probablt doing as much as any field of science to both use and confirm evolutionary theory.

But as I also explained, this is hard to put in a beginning high school biology book. It would be useful to see a summary of the role of embryological studies written so that at some point in a high school biology class students could get an understandable glimpse of this fascinating field.

Doc Bill · 30 July 2006

I'm sure that when my embryonic cells divided there was a huge debate between effing genius and flat feet.

I guess that explains my flat feet.

Jeremy Mohn · 30 July 2006

It would be useful to see a summary of the role of embryological studies written so that at some point in a high school biology class students could get an understandable glimpse of this fascinating field.
— Jack Krebs

As a high school Biology teacher, I second that notion. Ever since I finished reading "Endless Forms Most Beautiful," I have been looking for a way to make this topic more accessible for my students. Unfortunately, I don't think I understand evo devo well enough at this point to simplify it and still maintain strict scientific accuracy. Does anyone here know of any good resources?

Pete Dunkelberg · 30 July 2006

Read Coming to Life to connect the topic with down to earth biology. Then read Patterns and Processes of Vertebrate Evolution with a long chapter on evolution and development, still a very good deeper read despite how far both paleontology and evo-devo have come in ten years.

RBH · 31 July 2006

Pete wrote

Read Coming to Life to connect the topic with down to earth biology. Then read Patterns and Processes of Vertebrate Evolution with a long chapter on evolution and development, still a very good deeper read despite how far both paleontology and evo-devo have come in ten years.

Not to put the knock on those (or on Pete), but Jeremy and Jack likely have horrendous teaching loads. (My wife's a high school teacher.) Teaching five classes a day, with 30 or so students in each, doesn't leave a helluva lot of time to read and digest and prepare a high school level summary of material like that. It'd be real helpful to have that summary written by someone who doesn't face all those kids every day. RBH

Bruce Thompson GQ · 31 July 2006

On the other end of the spectrum, while speaking in Garden City, Kansas last Friday, a YEC in the audience hit me up with the "Haeckel's embryo's" issue.

Wouldn't a nice series of photographs showing gene expression illustrate the relationships much better? Labeled as different organisms they could serve a dual purpose and be used to illustrate the ID poof position. Of course when each organism appeared in the fossil record might pose a problem for the more conservative ID advocates but as with Pandas and People this could be glossed over so as not to offend some students. Or perhaps a special folding phylogenetic tree could be constructed like those in MAD magazine which would condense the time scale for some students and reveal a special ID tree, saving them from the evils of science. Delta Pi Gamma (Scientia et Fermentum)

james hannah · 14 November 2006

Is a female panda called a Plobajob??? My mate who is a male panda is wonderin...... cheers yes i know he is ah physco :(