Chapter 1: The Origins of Life and Cellular Complexity

Life on Earth began as solitary microscopic entities approximately 3.7 billion years ago. These simple cells endured until a transformative event occurred between 1.6 and 1.8 billion years ago, when two ancient microbes formed an unprecedented partnership. One of these was an α-proteobacterium, part of a vast and diverse group of bacteria known for performing various beneficial biochemical reactions. For example, the Rickettsia, a member of this group, includes notorious pathogens responsible for diseases like Rocky Mountain Spotted Fever.

Yet, not all α-proteobacteria are harmful; some exhibit cooperative characteristics. One particularly significant microbe was the ancient precursor to modern mitochondria, the essential metabolic “powerhouses” found in almost all eukaryotic cells—those that constitute multicellular organisms.

Due to the great age of this microbial union, the specifics surrounding the origin and identity of mitochondria's ancient ancestor remain somewhat elusive. Unlike dinosaurs, microbes do not leave fossils for researchers to analyze, compelling scientists to deduce this historical event through biochemical, genetic, and cellular methodologies.

This ancient microbial merger was vital. It not only initiated the evolutionary explosion of all multicellular life on Earth but also shaped fundamental processes in our biology, such as the biochemical pathways responsible for energy generation, which are crucial for various biosynthetic activities. Consequently, the identification of the last eukaryotic common ancestor (LECA) that led to modern eukaryotic cells with mitochondria has become a topic of significant interest.

To advance this research, it was essential to pinpoint which of the numerous α-proteobacteria could be the closest relatives of the ancestors of mitochondria. This challenge was taken up by biochemist Otto Geiger, a professor of ecological genomics at the National Autonomous University of Mexico, alongside his editorial role in the scientific journal BMC Microbiology.

In his recent study, Professor Geiger assembled a collaborative team that employed various biochemical, genetic, and cellular techniques to narrow down potential candidates to a select group of microbes that may represent the closest living relatives of the ancestral bacteria that evolved into mitochondria.

To achieve this, Professor Geiger and his team identified a dozen metabolic characteristics likely to be present in proto-mitochondria. They then surveyed all known genomes of α-proteobacteria—thousands in total—and compared their traits to the ones they selected.

However, this method faced a challenge: bacteria often engage in lateral gene transfer, redistributing genes across species, much like shuffling a deck of cards. Over time, this gene mixing results in unique combinations of genes and biochemical capabilities within different lineages, meaning no single α-proteobacterial lineage possesses the identical gene collection (and traits) that characterized proto-mitochondria 1.5 billion years ago.

A critical phase in this investigation occurred when Professor Geiger and his team focused on the capacity of potential ancestral α-proteobacteria to synthesize two essential lipid compounds that are vital for mitochondrial function. These lipids are unique, with one called ceramide exclusively produced by mitochondria, while the other, cardiolipin, plays a significant role in respiration and energy generation.

The identification of genes required for the synthesis of both lipids was crucial because they are involved in signaling when mitochondria are damaged and need to be removed—a function preserved in all eukaryotic cells. This finding suggests that the candidate proto-mitochondria likely possessed the necessary genes for lipid production.

But which modern α-proteobacteria have these lipid-synthesis genes?

Professor Geiger and his team discovered these characteristics in free-living marine α-proteobacteria prevalent in hot springs worldwide. Notably, this group had not previously been regarded as potential evolutionary ancestors of modern mitochondria.

Professor Geiger and his collaborators propose that these marine α-proteobacteria, belonging to the Iodidimonadales order, are strong candidates for representing modern relatives of ancient mitochondrial bacteria, as they exhibit traits associated with proto-mitochondria. The high oxygen gradient in their natural habitats further connects all Iodidimonadales, as these organisms rely heavily on oxygen for energy production, mirroring the function of mitochondria.

“These bacteria truly depend on oxygen,” Professor Geiger concluded, just as mitochondria do to generate energy.

Explore the fascinating role of mitochondria, often described as cellular parasites, in the evolution of complex life.

Chapter 2: The Path to Discovery

In this exploration of mitochondrial origins, researchers have made significant strides in understanding how these organelles evolved. The study of marine bacteria offers a unique perspective on the evolutionary history of mitochondria and their critical functions in modern cells.

Understand the evolutionary journey of mitochondria and their role in cellular energy production through the lens of marine bacteria.