Well, I’m into my second semester of graduate student life, and excited that my research is finally starting to get somewhere. Maybe. In theory.
I feel compelled to write about something I’ve noticed lately: science moves really fast. True, this may not come as a surprise to some of you, or even most of you. Maybe it’s because of my age, but even though I conceptually knew that science moves at a quick clip, it took a personal insight for me to really feel it. What follows a bit of a departure from my usual science-only schtick (I promise, there IS science here… it just needs an introduction, in the form of a grad school anecdote.)
All of the first year students in my department are required to take a course that meets weekly from 5 – 7 PM on Wednesdays. Although initially this served as a convenient means of getting to know each other and the faculty, now it just seems like a time sink. (I’d prefer to eat dinner instead.) Given my short attention span and general ornery demeanor on weeknight evenings, I did not expect to feel any sort of vast change in perspective in this class. So you can imagine my surprise when I did.
In lieu of remaining faculty to introduce ourselves to, the cohort has been discussing Steven Jay Gould’s book Wonderful Life: The Burgess Shale and the Nature of History. The details of the Burgess Shale may need to wait for another day–though it is a story worth telling (and perhaps not in Gould’s verbose style…)–but the book makes use of a staple of invertebrate biology: the arthropod family tree, formally called the phylogeny. That is where my real story begins.
The arthropods are the most numerous animals on the planet. Insects, crustaceans, spiders, horseshoe crabs, centipedes, millipedes, extinct trilobites, and assorted other creepy crawlies all fall into this gigantic phylum.
When Gould published his book, (the year I was born, as it turns out), the arthropod phylogeny was not well established, but the subgroups of arthropods were. The major groupings (I’ve omitted many smaller ones) looked something like this:
It was a fact. Insects were part of the group Uniramia with centipedes and millipedes. Crustaceans were a separate group.
Fast forward 22 years. At least one nerdy kid turned into a graduate student in evolutionary biology. There have been over seven generations of Game Boy. And insects are crustaceans.
What? Yes, you read that correctly. Even though Steven Jay Gould was absolutely confident in his declaration that insects were in a group with centipedes, (Uniramia) molecular evidence places insects WITHIN the Crustacea. And Uniramia doesn’t exist. Millipedes and centipedes are now in a group called Myriapoda.
But how could Gould have ever known? Most of the morphological evidence seemed to place insects away from crustaceans, in with centipedes and millipedes. They have different breathing structures (insects and centipedes have tracheae as opposed to crustacean gills). They have different limbs (crustaceans have biramous, two-branched limbs; insects and centipedes have uniramous limbs). Gould was partly right– it IS hard to pair insects with crustaceans on the basis of anatomy alone. By the late 1990s people began to suspect that the features linking the group Uniramia may have had multiple origins, but no one thought insects were nested within Crustacea. It’s taken a large body of molecular genetics research to start changing the accepted relationships among arthropods.
Of course, the first mention of the keyword “molecular genetics” that I can find in the Web of Science database was a paper by Linus Pauling in 1956. The idea of using molecules to find evolutionary relationships was initially proposed by Francis Crick in 1958, but wasn’t formally suggested until Emil Zuckerkandl and Linus Pauling published the idea in 1965. Willi Hennig’s foundational Phylogenetic Systematics, published first in 1969, doesn’t even really consider genetic methods for tree construction. Biologists created phylogenetic trees using protein sequences as early as 1967 before using genes, but even proteins were not used broadly until some time later.
In fact, it seems that molecular phylogenies were not commonly made or used until the 1990s–after Gould researched and published Wonderful Life. This is not entirely surprising. Polymerase chain reaction (PCR), a technique used to increase the quantity of DNA available for analysis, wasn’t invented until 1983–and was modified and improved through the 80s. (The process continues to this very day, making sequencing and amplification cheaper and faster). PCR was not a common procedure until the early 1990s. The molecular phylogenetic trees we see everywhere now were not available to Gould and his colleagues while he was writing; leaving them with only morphological, physiological, ecological, and behavioral traits to build their arthropod tree. No wonder they grouped insects with centipedes instead of crustaceans.
It thrills me to think of how much biology has grown in the past 30 years. Genetics has opened up a whole new world of questions, and new approaches to old ones. As much as Gould tends to decry technology’s ability to advance science, that this revolution happened over only 30 years is amazing. I can’t think of a single branch of biology that hasn’t been affected. And I don’t even use genetic methods myself.
So I find myself wondering: What’s going to happen in the next 30 years? How many of the things that we “know” now will change? Or be overturned completely? How many new things will we uncover? It’s a good time to be a scientist. If this much has happened in my lifetime thus far, I can’t wait to see what comes next.
Many thanks to my brilliant friend and colleague Bruno de Medeiros for sharing his insights on the history of the arthropod phylogeny, and providing comments on an early draft of this post.
The old phylogenetic classifications here I adapted from:
Erwin, DH. (1991) Metazoan phylogeny and the Cambrian radiation. Trends Ecol. Evol. 6: 131-134.
The new classification comes from an incredibly thorough phylogeny of extant arthropods:
Reiger, JC. et al. (2010) Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature. 463: 1079-1083.
** I added in trilobites as an unresolved group, because we’re lacking in arthropod phylogenies including both living and extinct groups.
Some of the history of PCR I learned from this paper:
Bartlett, JMS and Stirling D. (2003) A short history of the polymerase chain reaction. Method. Mol. Biol. 226: 3-6.