Are we human… or bacteria?

Various studies demonstrate that bacteria significantly outnumber the human cells in our bodies. So what does that make us? 

By Kaitlyn Liu

It’s strange to think that there are more foreign cells that inhabit our bodies than human cells. However, it’s not surprising considering bacteria are microscopic organisms that exist virtually anywhere in the world. From common places like your kitchen counter to harsh environments such as deep-sea hydrothermal vents, bacteria colonize every location one can think of.

Prokaryotic vs. Eukaryotic

The main thing that distinguishes our human cells from bacterial cells is that one has a nucleus while the other lacks one. Our cells, along with the cells of various other organisms, are eukaryotic; this means that we have membrane-bound organelles, such as the nucleus, which is surrounded by a nuclear membrane. On the other hand, bacteria are prokaryotes, which lack membrane-bound organelles and, subsequently, a true nucleus.

Prokaryotic cells are more primitive and were some of the first lifeforms to exist, dating as far back as 3.5 billion years ago—only a billion years after the formation of Earth’s crust. In contrast, the first eukaryotes did not appear until 2.7 billion years ago. Therefore, it could be inferred that prokaryotic cells were actually ancestors to eukaryotic cells.

But then… wouldn’t that mean that our cells evolved from bacteria? To answer that, we’ll have to start at the beginning. 

The Endosymbiotic Theory

The Endosymbiotic Theory is a theory that addresses how eukaryotic cells could have evolved from prokaryotic cells. Put simply, the word “symbiotic” refers to a relationship between two organisms in which both, or at least one, depends on the other for survival. According to the Endosymbiotic Theory, the mitochondria and chloroplasts, two organelles now found within eukaryotic cells, were once separate, prokaryotic entities.

Mitochondria can be found in human cells and are the “powerhouses” of the cell; they generate the chemical energy needed to fuel cellular processes. Similarly, aerobic bacteria, that is, bacteria that require oxygen to live, produce energy in the same way. Furthermore, blue-green bacteria and chloroplasts (found in plants) possess identical types of chlorophyll, a pigment essential to photosynthesis.

Although there are several other pieces of evidence to suggest that structures within our cells may have once been bacteria, the most significant proof is the presence of a double membrane. Mitochondria and chloroplasts both possess double membranes, which were likely developed from the membrane of the ancestral eukaryotic cell. In other words, billions of years ago, our cells engulfed a prokaryotic organism and incorporated it as part of its cellular functions.

Essentially, human cells only developed some of the functions they possess today due to endosymbiosis with bacteria. However, not only did our cells coexist with prokaryotes in the past, but our bodies still form present-day symbiotic relationships with bacteria.

Gut Microbiome

The microbiome is the collective community of microorganisms that inhabit our bodies. While one typically generalizes all bacteria as harmful, in actuality, many of these microscopic organisms are good for our health. Most of these beneficial bacteria exist within our guts and perform a variety of functions, including:

• Breaking down food
• Producing important nutrients, such as Vitamin K
• Regulating the immune system
• Protecting against pathogens

In addition, recent studies have shown correlations between an unhealthy gut microbiome and increased risks of Alzheimer’s, among various other diseases. With this information, it is not inaccurate to assume that our wellbeing depends on bacteria.

Bacteria or Human?

So what does that mean for us? After all, bacteria are not only a fundamental part of our cellular processes, but they also substantially contribute to our health. Nevertheless, while we are most definitely human since we differ from bacteria at the cellular level, the fact remains that we were, and still are reliant on these prokaryotes.