American Scientist

To the Editors:

In his timely column “Evolution’s Other Narrative” (November–December, Macroscope), Bradford Harris puts forward the concept of symbiosis, “cooperation and interdependence,” as an important contributor to evolutionary processes that is distinct from the widely accepted neo-Darwinian, gene-driven “survival of the fittest.”

Harris is admirably critical of the genetic blueprint explanation of evolution. However, ascribing to microorganisms the capacity for “cooperation and interdependence” is factually incorrect. Commensalism (eating at the same table) is the preferred concept used by scientists collaborating on the Human Microbiome Project.

The concept of commensalism is distinct from symbiosis and mutualism; it does not imply interdependence. Rather, commensals are a population of obligate bacteria, viruses, yeasts, and other microbial life whose life cycle turns over rapidly and who promiscuously swap and integrate genetic information via horizontal and vertical transfer. The commensals that make up the gut flora (and the wider microbiome) are not cooperating with one another. They are annexing each other’s genes, competing for limited resources and “trading” the products of their metabolism.

Although constituting a balanced and robust ecosystem, when conditions fluctuate, commensals can adapt—fast.

Christine McNulty
Oxhey, Hertfordshire, United Kingdom

To the Editors:

I was shocked to read in “Evolution’s Other Narrative” author Bradford Harris’s statement that Ivan Wallin of the University of Colorado had “proposed a radical hypothesis” in the 1920s that mitochondria were the descendants of bacteria.

I learned in medical school in the United Kingdom in 1965 that the idea was proposed—to the ridicule of his colleagues—by German pathologist Richard Altmann in the 1890s. The stain that he developed for mitochondria still carries his name.

This story is more recently told by Brian O’Rourke in a 2010 paper in Frontiers of Physiology, where he quotes Altmann as saying that bacteria were “autonomous elementary organisms.”

We medical students were also told that Altmann became depressed because of the rejection of his ideas and then committed suicide, but I am not sure if that was simply hearsay. His biography on Wikipedia states only that he died of a “nervous disorder.”

Poor Altmann is again denied recognition in your recent column.

Jonathan Pulman
Ottawa, Canada

Mr. Harris responds:

I am happy to hear Dr. Pulman acknowledge that Ivan Wallin’s work was in line with previous work illuminating the bacterial ancestry of mitochondrial structures. I was not attempting to argue in the column that the idea of mitochondria’s bacterial ancestry originated with Wallin, only that he pushed that idea forward with a new set of direct genetic evidence—just as the intensity of scientific focus on nuclear genetics swelled. I chose to present Wallin in particular because, given my length constraints, Wallin’s work was the most conspicuous early 20th-century example of evolutionary research that contradicted the mainstream nuclear genetic focus, and because Wallin’s work was among the most inspiring to evolutionary biologist Lynn Margulis.