Wednesday, April 8, 2009

Cautious Darwin, a Fossil Sneeze, and the Real Octomom

Charles Darwin was a cautious scientist. More than 20 years passed between the conclusion of his voyage on the Beagle, during which he made many of the observations that would impel the development of his theory of natural selection, and the publication of his world-transforming work, "The Origin of Species". Darwin understood the potential trouble posed by the iconoclastic things he had to say, both with respect to his personal life - his wife Emma was a devout Christian - and with respect to his standing in the scientific community.

Although we imagine now that Darwin had "nailed it" when he published "Origin" in 1859, there were significant elements of his nascent theory that were subject to reasonable challenge. For one thing, Darwin had no knowledge of the mechanisms of inheritance upon which his process of natural selection relied. It would be half a dozen years before Gregor Mendel's discoveries involving plant hybridization would be published, and these would go largely unnoticed for another three decades. How, indeed, were "favored traits", so central to Darwin's hypothesis, transmitted from generation to generation? Without anything like a theory of genetics, Darwin hadn't a clue.

To make matters worse, Darwin was painfully aware of the problems presented by what he called "the imperfection of the geological record", enough so that the topic merited a chapter of its own in his book. The absence of fossil evidence for transitional species - so-called missing links - was a vulnerability that Darwin addressed as best he could. Today we can make allowances for the fact that the collection and identification of paleontological specimens was then still in its infancy as a systematic enterprise - a defense that was hardly available to Darwin at the time. Much to the chagrin of creationists, who - contrary to the wealth of discoveries made in the last 150 years - insist that these deficits in the fossil record remain, few missing-links have gone missing. In fact, the contemporary fossil record is rich in finely graduated intermediate forms.

It does turn out that Darwin was wrong about one claim he made in "Origin" with regard to fossilization, one that underscores his cautious expectations. In enumerating the many limitations of geological evidence, he despaired that "no organism wholly soft can be preserved." Shells and bones are all that we could count on, according to Darwin, and even they "can disappear when left on the bottom of the sea".

Meet Keuppia levante, a 95-million-year-old octopus, whose fossilized remains were recently discovered by Dirk Fuchs and fellow researchers from the Freie Universit├Ąt Berlin and announced March of this year. Since octopuses consist pretty much of muscle and skin, what isn't eaten almost immediately by scavengers upon their demise, decays rapidly into a blob of slime. The odds of finding a fossil octopus have been compared to those of finding a "fossil sneeze". Looking at this Cretaceous-era octopus, so exquisitely preserved in Lebanese limestone, it would appear it is about time - finally - for someone offer an appropriate "fossil gesundheit".

What is, perhaps, more remarkable, according to Fuchs and his colleagues, is how closely this ancient specimen resembles its present-day descendants. Such invariance in form would have come as a surprise to Darwin, since his conception of evolution by natural selection imagined the accumulation of favored differences - gradually, but inexorably - over time. To him, the probability of a particular species remaining largely unmodified over the eons of geological time would have seemed to have been astronomically small.

(As an aside I'll note that this "error" on Darwin's part serves to remind us that, although his were the first words in the theory of evolution, they are certainly not the last. Unfortunately, there is a tendency in some quarters to equate Darwin's hypotheses in "Origin" with our current understanding of the process of natural selection. Enough confusion exists that some suggest, "let's get rid of Darwinism," declaring to the world, in all deference to the master, that we have, in the last 150 years, moved significantly beyond many of his original proposals. I think they have a point.)

But, although Darwin could possibly be faulted for not anticipating that, by dint of the hard work of 21st century paleontologists such as Dirk Fuchs, fossil sneezes, such as Keuppia levante, would ultimately be unearthed, there is a method available today to evolutionary science to probe the biological past that he could never have anticipated in his wildest dreams - namely, the analysis of the DNA of living creatures.

With this in mind, allow me to introduce Megaleledone setebos or, as I call her, the real octomom.

Thirty million years ago octomom or, more accurately, members of a species quite similar to hers, roamed the waters surrounding Antarctica. Sea ice was, for the first time, massing on the surface, removing fresh water from the Southern Ocean and leaving below it a highly saline environment enriched in oxygen - a fitting octopus habitat. As the climate continued to cool and the Antarctic ice shelf grew, streams of these salty, oxygenated waters flowed northward along the ocean floor carrying with them octomom's sisters and cousins, who would, themselves, become the founding mothers of new, distinct deep-water octopus species in other parts of the globe.

How do we know the details of this octopus "out of Antarctica" saga? Since they are so exceedingly rare, scientists couldn't use fossilized octopus remains to reconstruct this prehistoric exodus, so, absent a fossil record, they turned to the genetic one, the record written in the DNA of the cells of octopuses living today.

Thanks to the work of the Census of Marine Life, a decade-long project begun in 2000 "to assess and explain the diversity, distribution, and abundance of marine life", specimens representing a number of the living species of deep-sea octopuses were collected and delivered to researchers at Queens College in Belfast. There, by analyzing extracted DNA, biologist Jan Strugnell was able to formulate a family tree for these creatures which not only demonstrated the extent they were related to one another, but also resulted in a calculated genetic profile for their common ancestor, who, although 30 million years older, turns out to be a dead ringer, so to speak, for Megaleledone setebos, our very own octomom.

Darwin's misstep in doubting the possibility of the fossilization of organisms consisting almost entirely of soft tissue is quite understandable. How could he have anticipated the discovery of a fossil sneeze? His concerns about the deficiencies in the geological record have been - and continue to be - addressed and rectified by the diligent work of paleontologists, who, from all appearances, leave no stone unturned. To augment these traditional methods of evolutionary investigation researchers use the tools of molecular biology and genetic science now at their disposal. These allow them to peer into the deep time of the history of life on earth using DNA from the cells of living organisms - a window into the past that Darwin could not have imagined.

All said, Darwin would, no doubt, be astounded by and proud of what his scientific heirs have made of the simple - and cautious - beginnings of his theory of evolution by natural selection.

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Cautious Darwin, a Fossil Sneeze, and the Real Octomom by Marc Merlin is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.
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