Sometime Shortly After Abiogenesis
The Great Leap Forward for Energy Metabolism
Abiogenesis has been a sticking point for naturalists and creationists in the argument over the necessity for a creator. Biologists are often careful to point out that evolution and natural selection begin where abiogenesis sets the table, and I disagree on that. I have maintained that the process of abiogenesis was a matter of a continuum of change in chemistry over a period of millions of years way back about 4 billion years ago and that the whole process wavered back and forth over some threshold before natural selection, adaptation and flow among proto-life and life finally took hold.
It was all an evolutionary process, as various means of energy metabolism proved stronger and more successful than others, leading to a strategy that allowed self-sufficiency and the maintenance of individual self-contained cells that we would all agree are living. The definition of “life” is fuzzy, so much so that the argument belongs as much to philosophers of science as it does to biologists.
So when did life begin? We will never be able to pinpoint a fossil or genetic “first life,” but we can say a few things about what is common to all life here and now. Cells have membranes to contain all of their functioning parts, filter those molecules that should enter from those that should stay outside. Cells have DNA, which provides the framework for the proteins and organelles which make up the cells. Cells have RNA, which carries the message from the DNA through the cytoplasm to determine the characteristics of the organelles within the cytoplasm. This, of course, is a very general description.
Cells also contain mitochondria, and this is one of the puzzles of early evolution. Mitochondria are symbiotic, originally a form of self-sufficient bacteria, which when integrated with eukaryotic cells, gain and provide benefit to the host cell. Mitochondria have their own set of DNA, and this DNA provides the basis for some of its key structures, and the unique tools that the mitochondria use to convert sugar into the energy it shares with its host.
As a symbiotic feature, mitochondria have safe homes inside the cells, and they pay rent by using the Krebs Cycle to provide much more energy than the hosts would be able to produce without them. Mitochondria are perhaps the most successful example of how cells work in tandem to assure mutual survival. Mitochondrial DNA is also passed, in sexually reproducing life, through the female’s DNA line, which yields data on lineage. This is unique and helpful, but not the issue in this post. I only mention it here because this is the way that mitochondria have entered the vernacular. I am sure that my readers are aware of the “Mitochondrial Eve,” a woman who lived on the order of 200,000 years ago and is thought to be the ancestor of all humans now alive.
My question remains on the origin of the mitochondrial integration with eukaryotic cells. A recent commenter made the claim that evolution can’t explain how mitochondria came to be included in the cellular structure of eukaryotes, and I did some checking. Guess what I found with a quick search through PubMed? Don’t sweat it too much, cause I’ll just blurt it out. I found a paper from 2001 that explains the likely integration. I just wish creationists wouldn’t get so cocky, because it is so easy to burn down their strawmen.
Complete sequences of numerous mitochondrial, many prokaryotic, and several nuclear genomes are now available. These data confirm that the mitochondrial genome originated from a eubacterial (specifically α-proteobacterial) ancestor but raise questions about the evolutionary antecedents of the mitochondrial proteome.
Recent debates about eukaryotic cell evolution have been closely connected to the issue of how mitochondria originated and have evolved [1,2,3,4,5,6,7]. These debates have posed such questions as the following: Did the mitochondrion arise at the same time as, or subsequent to, the rest of the eukaryotic cell? Did it originate under initially anaerobic or aerobic conditions? What is the evolutionary relationship between mitochondria and hydrogenosomes (H2-generating and ATP-producing organelles that are found in eukaryotes lacking mitochondria)? Is the amitochondrial condition in these organisms a secondary adaptation or is it evolutionarily primitive – or, in other words, did any organisms diverge from the main line of eukaryotic evolution before the advent of mitochondria? Whereas the issue of how the eukaryotic cell arose remains controversial [8,9], current genomic data do allow us to make a number of reasonably compelling inferences about how mitochondria themselves originated and have since evolved.
Now, I have a confession to make on this paper. Because I am not a trained biologist, I only understood portions of the study. I read through it and ran into many concepts with which I am not familiar. So why would I write about this?
I wanted to point out that an important pathway leading to the emergence of eukaryotes from their prokaryotic ancestors includes the integration of mitochondria, and in this study, the authors examined the relationships between the genomes of yeast and mitochondria to show how this integration likely occurred. Examination of genomic sequences has become nearly as important as the study of fossils into determining the early pathways of evolution, and it will likely assist biology in answering these questions to a degree that paleontology can’t match.
I don’t mean to say that examining genomes can replace paleontology. I mean that they are complementary tools of science.
Further, since we all as eukaryotic beings carry mitochondria in all of our cells (even sperm cells carry a tiny fragment of mitochondrial DNA), we have an answer to the creationist objection, “Were you there?”
We can answer, “Yes, we were there. Almost. We were there sometime shortly after abiogenesis.” Yes, it was likely some hundreds of millions of years after life passed the abiogenesis threshold once and for all, but in the geological timeframe, it was a very short time indeed.
Okay, so today is Charles Darwin’s birthday. The claim is that natural selection only earned its wings once the first cell arrived and began its differential reproductive success. This “natural selection” thing and the publication of On the Origin of Species are well worth celebrating, because although there had been sketches and inklings prior to Darwin’s formulation, there was no serious explanation for how evolution could work. Darwin broke the ice, so to speak, and encouraged biologists to explore natural means of species creation.
This is why we celebrate Darwin. His work was not the beginning of the idea of evolution, and more importantly his work was not the end nor the summit of evolutionary thought. We celebrate because he took the first steps of serious investigation into the pathways of evolution. He showed us the tools that we could use to explore our common heritage in a way that provides answers—and questions.
This geologist was the Newton of biology. Just as our understanding of physics has grown immensely since Newton, so to our understanding of biology has bloomed since Darwin. His was the seed that led to the flowering and the study of such tools as genomics. Funny that he wasn’t even aware of genetics, but there you have it. And so do we.
Genome Biol. 2001; 2(6): reviews1018.1–reviews1018.5.Published online 2001 June 5.PMCID: PMC138944Copyright © 2001 BioMed Central LtdThe origin and early evolution of mitochondriaCorresponding author.
Michael W Gray: M.W.Gray@dal.ca
This entry was posted on Thursday, February 12th, 2009 at 7:43 am and is filed under Mike Haubrich, Science. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.