Why Mitochondria are Not Found Outside Extant Living Systems

A living system requires power. To be outside a living system is unnecessary for mitochondria. The rationale behind this can be explored through both scientific understanding and theoretical speculation. According to the endosymbiotic theory, mitochondria may have evolved from independent organisms absorbed by larger, more complex creatures over time.

Paul R. Rothman, Senior Department of Cell Molecular Medicine at Johns Hopkins University, suggests that mitochondria may have once been a unique, separate power generating organism. Over time, this organism was likely absorbed by a larger creature. This evolutionary path has led to the mitochondria being an essential part of eukaryotic cells today, rather than an independent entity.

Evolutionary Absorption and Endosymbiosis

The concept of endosymbiosis, as proposed by NASA Astrobiology Institute and the University of Arizona, is crucial here. In the early stages of cellular evolution, it is theorized that bacteria-like organisms invaded eukaryotic cells and became the ancestors of modern mitochondria. This process involved the invasion of smaller cells into larger ones, leading to a mutually beneficial relationship. Over time, these invading cells evolved into the complex organelles we know today.

Theoretical Perspectives on Mitochondrial Origins

Scientists and theorists propose that complex life forms could have developed through multiple evolutionary pathways. In this context, it is interesting to consider what might power life in creatures composed of different elemental compositions. For example, nitrogen-based or silicon-based life forms could potentially generate power through entirely different mechanisms, possibly independent of what we know as mitochondria.

These hypotheses are part of the ongoing scientific inquiry into the origin and evolution of life. Different perspectives may exist, and further research is needed to fully understand these processes.

Current Independent Organisms

While some bacteria, such as those in the family Anaplasmataceae belonging to the Proteobacteria order, are similar to mitochondria in function, they are not direct descendants. These bacteria can live independently in the wild, but they are not the same as the mitochondria found in eukaryotic cells. The ancient mitochondrion in the ancestors of eukaryotic organisms, including humans, absorbed these bacteria about 2 billion years ago, making them radically different from their prokaryotic cousins today.

Therefore, it is unlikely to find independent organisms on Earth today that exactly match current or ancient human mitochondria. The independent surviving organisms, if they exist, might be found in extreme environments, such as deep in the Earth's crust or other challenging conditions.

Evolutionary Changes Over Time

Human mitochondria have undergone significant evolutionary changes over the 2 billion years since their absorption into eukaryotic cells. Mitochondria have lost many of their own mitochondrial genes and functions, transferring many to the cell's own DNA located in the nucleus. This ongoing process of adaptation and evolution further distinguishes modern mitochondria from their bacteria-like ancestors.

Studies continue to reveal the intricate details of these evolutionary changes, shedding light on how mitochondria have become an integral part of eukaryotic cells, rather than remaining as independent organisms.

Thus, while the theoretical possibility exists that similar power-generating organisms could exist in independent form, the likelihood of finding such organisms on Earth, especially those matching current human mitochondria, is very low.