Some called her a scientific heretic. Others were a bit more kind, describing her as a maverick.
Lynn Margulis (1938–2011) earned her reputation in the late 1960s when she proposed the endosymbiont hypothesis for the origin of eukaryotic cells. Because her ideas about evolution didn’t conform to Darwinian principles, evolutionary biologists summarily dismissed her idea out of hand and then went on to ignore her work for a couple of decades. She was ultimately vindicated, however, as the endosymbiont hypothesis gradually gained acceptance.
Today, Margulis’s proposal has become a cornerstone idea of the evolutionary paradigm and is taught in introductory high school and college biology courses. This classroom exposure explains why I am often asked about the endosymbiont hypothesis when I speak on university campuses. Many first-year biology students and professional life scientists alike find the evidence for this idea compelling, and consequently view it as providing broad support for an evolutionary explanation for the history and design of life.
Yet, new work by biochemists from Cambridge University make it possible to account for the origin of eukaryotic cells from a creation model perspective, providing a response to the endosymbiont hypothesis.1
The Endosymbiont Hypothesis
According to this hypothesis, complex cells originated when symbiotic relationships formed among single-celled microbes after free-living bacterial and/or archaeal cells were engulfed by a “host” microbe. (Ingested cells that take up permanent residence within other cells are referred to as endosymbionts.)
Accordingly, organelles, such as mitochondria, were once endosymbionts. Once taken inside the host cell, the endosymbionts presumably took up permanent residency within the host, with the endosymbionts growing and dividing inside the host. Over time, the endosymbionts and the host became mutually interdependent, with the endosymbionts providing a metabolic benefit for the host cell. The endosymbionts gradually evolved into organelles through a process referred to as genome reduction. This reduction resulted when genes from the endosymbionts’ genomes were transferred into the genome of the host organism. Eventually, the host cell evolved both the machinery to produce the proteins needed by the former endosymbiont and the processes needed to transport those proteins into the organelle’s interior.
Evidence for the Endosymbiont Hypothesis
The main line of evidence for the endosymbiont hypothesis is the similarity between organelles and bacteria. For example, mitochondria—which are believed to be descended from a group of α-proteobacteria—are about the same size and shape as a typical bacterium and have a double membrane structure like gram-negative cells. These organelles also divide in a way that is reminiscent of bacterial cells.
There is also biochemical evidence for the endosymbiont hypothesis. Evolutionary biologists view the existence of the diminutive mitochondrial genome as a vestige of this organelle’s evolutionary history. Additionally, the biochemical similarities between mitochondrial and bacterial genomes are taken as further evidence for the evolutionary origin of these organelles.
The presence of the unique lipid called cardiolipin in the mitochondrial inner membrane also serves as evidence for the endosymbiont hypothesis. Cardiolipin is an important lipid component of bacterial inner membranes, yet it is not found in the membranes of eukaryotic cells—except for the inner membranes of mitochondria. In fact, biochemists consider it a signature lipid for mitochondria and a vestige of this organelle’s evolutionary history.
A Creation Model Perspective on Mitochondria
So, as a creationist, how do I make sense of the evidence for the endosymbiont hypothesis?
Instead of focusing my efforts on refuting the endosymbiont hypothesis, here, I take a different approach. I maintain that it is reasonable to view eukaryotic cells as the work of a Creator, with the shared similarities between mitochondria and bacteria reflecting common design rather than common descent.
However, to legitimately interpret mitochondrial origins from a creation model perspective, there must be a reason for the biochemical similarities between mitochondria and bacteria. Previously, I wrote about discoveries that provide a rationale for why mitochondria have their own genomes. (See “Resources.”) Thanks to recent research advances, an explanation now exists for why the mitochondrial inner membranes harbor cardiolipin.
Cardiolipin’s Function
Previous studies identified close associations between cardiolipin and a number of proteins found in the mitochondrial inner membrane. These proteins play a role in harvesting energy for the cell to use. Compared to other lipid components found in the inner membrane, cardiolipin appears to preferentially associate with these proteins. Evidence indicates that cardiolipin helps to stabilize the structures of these proteins and serves to organize the proteins into larger functional complexes within the membrane.2 In fact, several studies have implicated defects in cardiolipin metabolism in the onset of a number of neuromuscular disorders.
The work of the Cambridge University investigators adds to this insight. These researchers were using computer simulations to model the interactions between cardiolipin and a protein complex called F1-F0 ATPase. Embedded within the inner membrane of mitochondria, this complex is a biomolecular rotary motor that produces the compound ATP—an energy storage material the cell’s machinery uses to power its operations.
Like other proteins found in the inner membrane, cardiolipin forms a close association with F1-F0 ATPase. However, instead of permanently binding to the surface of the protein complex, cardiolipin dynamically interacts with this membrane-embedded protein complex. The researchers think that this dynamic association and the unusual chemical structure of cardiolipin (which gives it the flexibility to interact with a protein surface) are critical for its role within the mitochondrial inner membrane. As it turns out, cardiolipin not only stabilizes the F1-F0 ATPase complex (as it does for other inner membrane proteins), but it also lubricates the protein’s rotor, allowing it to turn in the viscous cell membrane environment. Also, its unique structure helps move protons through the F1-F0 ATPase motor, providing the electrical power to operate this biochemical motor.
The bottom line: There is an exquisite biochemical rationale for why cardiolipin is found in mitochondrial inner membranes (and bacterial membranes). In light of this new insight, it is reasonable to view the shared similarities between these organelles and bacteria as reflecting common design—the product of the Creator’s handiwork. Like most biological systems, this organelle appears to be designed for a purpose.
Anna Duncan, Alan Robinson, and John Walker, “Cardiolipin Binds Selectively but Transiently to Conserved Lysine Residues in the Rotor of Metazoan ATP Synthases,” Proceedings of the National Academy of Sciences USA 113 (August 2016): 8687–92, doi:10.1073/pnas.1608396113.
Giuseppe Paradies et al., “Functional Role of Cardiolipin in Mitochondrial Bioenergetics,” Biochimica et Biophysica Acta—Bioenergetics 1837 (April 2014): 408–17, doi:10.1016/j.bbabio.2013.10.006.
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