Microbes
Microbes All The Way Down?
Carl Zimmer offers a quick look at an alternative theory being put forward by James Lake, a University of California - LA researcher, concerning the origins of complex cells in early life forms. First a bit of background from Zimmer:
We are, fundamentally, a fusion. As I wrote in my essay for Science on the origin of eukaryotes, there's now a wealth of evidence that our cells evolved from the combination of two different microbes. The mitochondria that generate fuel for our cells started out as free-living bacteria. Today, they still retain traces of their origin in the bacterial DNA they carry, as well as their bacterial structure, including the membrane within a membrane that envelops them.
Scientists I spoke with as I worked on the essay agreed that this merging was a profound event in the history of life. No living eukaryote, whether animal, plant, fungus, or protozoan, has completely lost its mitochondria since that symbiotic milestone some 2 billion years ago. It wasn't the only time that two species merged, however. Plants, for example, descend from algae that engulfed a species of photosynthesizing bacteria. Many protozoans have swallowed up photosynthetic partners as well.
Yet in all these cases, eukaryotes did the swallowing. It's striking that scientists have such a hard time finding an example of a noneukaryote (a prokaryote such as Escherichia coli and other bacteria) hosting a prokaryote symbiont. Some scientists have gone so far as to argue that swallowing up a partner requires lots of intricate molecular systems that can create a pocket in the surface of a cell and can draw that pocket inside the cell as a bubble. Eukaryotes have this sort of cellular skeleton, and prokaryotes, it seems, don't. If that's true, then our ancestors swallowed up mitochondria only after they evolved the molecules necessary for the swallowing.
I'm fascinated by the symbiotic mergers of two or more life forms to create new life forms or enhance existing ones. Studies of Mitochondria have delivered some of the strongest evidence that this has occurred, leaving a natural history of symbiotic interaction in most eukaryotic cells. Mitochondria possess their own DNA which is only passed down through maternal inheritance (from the female in each species). I imagine that differences in mitochondria found in various species alone could occupy the careers of myriads of cell biologists. For example:
In animals the mitochondrial genome is typically a single circular chromosome that is approximately 16-kb long and has 37 genes. The genes while highly conserved may vary in location. Curiously this pattern is not found in the human body louse (Pediculus humanus). Instead this mitochondrial genome is arranged in 18 minicircular chromosomes each of which is 3–4 kb long and has one to three genes. This pattern is also found in other sucking lice but not in chewing lice. Recombination has been shown to occur between the minichromosomes. The reason for this difference is not known.
Anyway, the meat of Zimmer's post:
But today, there's a provocative new alternative to consider. Maybe a lot of today's prokaryotes are also the result of an ancient merger. The idea comes from James Lake of the University of California, Los Angeles, a veteran researcher on the early history of life. In my essay, I describe how Lake first proposed in the early 1980s that the host cell that gave rise to eukaryotes belonged to a lineage of prokaryotes he dubbed eocytes. Now, a quarter of a century later, new studies on genomes are strongly supporting his eocyte hypothesis. In today's issue of Nature, Lake questions whether we may be too quick to assume that only eukaryotes are the result of fusion. He observes that aphids depend on a species of bacterium called Buchnera to digest their food, and Buchnera in turn contains other bacteria on which its own survival depends. These two bacteria are still distinct enough from each other that we can tell them apart. But what if two bacteria joined together billions of years ago and their identities blurred together? How would we tell them apart?
To look for possible signs of ancient fusion, Lake compared proteins in over 3000 different prokaryote genomes. He concluded that a major group of bacteria known as Gram-negative bacteria is actually the result of a fusion of two different kinds of bacteria, known as Actinobacteria and Clostridia. These bacteria, which include the ancestors of mitochondria, are unusual in many ways, but the most obvious one is their membranes. Whereas other bacteria are surrounded by a single membrane, Gram-negative bacteria are surrounded by two. It's possible, Lake argues, that the double-membrane structure of these bacteria is a vestige of one kind of bacteria living inside another.
Book Review: Darwin's Blind Spot: Evolution Beyond Natural Selection by Frank Ryan
(out of 5 stars)
I picked up this book hoping that its subtitle, 'Evolution Beyond Natural Selection' would offer an interesting abstration of neo-Darwinian theory. In some ways, it certainly did so, but the results are a strange mix of solid science, sour grapes, and wishful cheerleading.
Make no mistake, the book is a relatively easy read, is well-written, and contains a ton of reference to good science, scientists, and some of the latest arguments on all sides of evolutionary thought. The backbone of the book, its discussion of symbiosis, is fascinating and would offer enlightenment to any reader unfamiliar with it. Ryan does an outstanding job conveying the importance of symbiotic relationships throughout most of the book, and I have no doubt of its importance as a primary driver of evolutionary diversity.
He hits at neo-Darwinists repeatedly, although generally backed up by some science. He uses Darwin's last 2 points (the gradual shift of the genome through small changes) to hammer away at neo-Darwinian theories, and offers some solid proof that Darwin was wrong to deny that evolution could be nearly spontaneous and dramatic. However, somewhere around halfway, I began to have some doubts about where this was heading. He had mentioned Lovelock and Gaia Theory, mostly in passing to that point, but had not quite dove head first into the fray. But his insistence throughout on emphasizing the positive aspects of symbiosis as key to evolution led to the utter decline of his narrative into an argument in favor of altruistic behavoir as something other than the widely accepted selfish gene theory. That's ok by itself, alternative theories offer guides toward better understanding, but the last 3 chapters are quite bizarre compared to the rest of the book. Ryan steps into near-metaphysical conjectures about altruism, social acceptance, and several times cherry-picks bits of good science to argue his points. Several alternative theories to selfish genes are thrown out, almost in a scattershot approach, kind of a last ditch effort to convince the reader that the neo-Darwinians are wrong, or at best, misleading.
I'm open to some of the alternative theories Ryan offers, clearly what we know about evolution is not the full picture and the role of symbiosis has been largely undervalued in popular science books, but by the time I'd read the first 80% of the book, I'd drawn a conclusion based on his presented evidence that the selfish gene theory could explain almost everything he claimed Darwinians could not explain. A few times he even props up stale straw men just to knock them down with a less-than-stellar argument in favor of his view.
At the end of the read, it was worth the time despite its weaknesses, and it should be an excellent introduction to the role of symbiosis in evolution and in daily life. Just consider how little direct evidence he offers for his secondary thesis that selfish gene theory fails to explain some of the things he offers, and I personally ignored his insistence in using Gaia as his platform for discussion. Altruism is a big one and Ryan's treatment of it really soured this book a bit at the end given that he had fallen down into subjective speculation rather than solid science. Take note of his leap-of-faith trick in refering to the 3-million year old footprints of (I believe it was Australopithecus africanus) in his discussions of social altruism. Overall, three and one-half stars.
Book Review: Liaisons of Life: ... How the Unassuming Microbe has Driven Evolution
(out of 5 stars)
I've been on a kick lately learning about Dawkin's Extended Phenotype and the idea that genetics can drive a complex system in ways that aren't solely isolated to a single body vehicle. I thought this book might take up that idea and run with it, but the author doesn't quite make that connection to dawkins. However, he comes very close with Liaisons. This book is a quick read and includes examples of microbes that have joined larger plants or animals to become symbionts. Most of the examples are of mutually beneficial relationships between the microbes and their hosts, including lichen, angler fish, legumes, and many others.
This is an area of science that is fascinating, quickly changing, and can be mind boggling at times, but I felt Wakeford did an excellent job conveying his points and explaining the complex relationships without overburdening with deep science. There is good detail in the book, and it is not for the very casual science reader, but still approachable by curious readers. Recommended for anyone seeking an extention of the Phenotype idea or general science readers. Four stars.
