Cell Structure: The things that live inside us

Many of us are familiar with the basics of cellular structure. Cells have genetic material, organelles that perform specific functions, and cytoplasm, the gel-like fluid which sustains many of the complex chemical reactions going on in our cells at any given moment. 

But how did these structures emerge? In terms of evolution, scientists can look at the genome of living things to compare which of these structures are shared and which emerged first.

Interstingly, there is a key difference between the cell structures of prokaryotes (single-celled organisms like bacteria and archaea) and eukaryotes (everything else; humans, plants, fungi, etc.). Whereas eukaryotes produce ATP (the energy currency of life) in their mitochondria, prokaryotes do not, producing it on their cell surface membrane. Even more interesting, prokaryotes completely lack mitochondria. So where did this cellular structure come from, then?

Well, the answer may sound like science fiction, but it is currently the most accepted theory for how mitochondria emerged in eukaryotic cells. 

The first key piece of evidence is that mitochondria have their own DNA, unlike other organelles. The mitochondrial DNA is unlike that of the DNA in our own cell's nucleus. Our DNA is organised linearly, whereas mitochondrial DNA is organised in a ring, circularly. This leads to the question, what other organisms organise their DNA this way? Primarily, prokaryotes, like bacteria, are known for their circular DNA. This fact, coupled with the prokaryote's own lack of mitochondria, points to the cellular structure's origin. 

Scientists suggest that at some point, an early ancestor of eukaryotes engulfed another form of life resembling that of modern-day prokaryotes. Our ancestral cell would have recognised the inherent benefit of supporting another form of life within its cell walls. While the eukaryotic cell could focus on carrying out its own functions like DNA replication, the other cell could be coopted to produce ATP for both organisms in return for being protected by the eukaryotic cell. 

This form of symbiosis was obviously beneficial, as now all eukaryotes have mitochondria within their cells. In this manner, the ancestor of our own mitochondria were free-floating organisms that our cells engulfed and then recognised as beneficial in a process named symbiogenesis. Several scientists developed this idea over time, but the evolutionary biologist Lynn Margulis was the first modern scientist to popularise and substantiate the idea after initially receiving heavy criticism from the scientific thinkers of the day.