Researchers stared at organisms too small to see as they leaned over screens glowing with genetic sequences in a lab overlooking the Texas coastline, where shallow seawater changes from muddy brown to dull green. The low hum of machines processing microbial DNA was the only sound in the room. Considering what they’re actually looking at—the potential ancestors of everything alive today—it’s difficult to ignore how serene the moment feels.
For billions of years, there was little life on Earth. In rocks, sediments, and oceans, bacteria and archaea flourished, surviving effectively without ever becoming complex. Evolution appeared to be at a standstill for a considerable amount of time. No matter how long they existed, these early cells never passed the imperceptible threshold into complexity because they had limitations, particularly energy limits.
There had to be a change.
Scientists now believe that what sounds more like an accident than a grand design was the pivotal event. One microorganism ingested another. A simple act of predation would normally have been the end of the story. However, the swallowed cell was not digested in this unique instance. It lived on, becoming an unanticipated tenant of its host.
| Category | Information |
|---|---|
| Scientific Event | Endosymbiosis: one microbe living inside another |
| Approximate Time | Around 2 billion years ago |
| Key Players | Asgard archaea and alphaproteobacteria |
| Result | Formation of mitochondria inside complex cells |
| Importance | Enabled energy surplus for complex, multicellular life |
| Recent Breakthrough | Discovery that Asgard archaea could tolerate oxygen |
| Reference | Nature Journal – Evolution of Complex Life Research |
| Reference | SciTechDaily – Microbes and Origin of Complex Life |
What happened next might not have been intended by either organism.
The smaller microbe started producing power inside its host because it could use oxygen to create a lot of energy. Rather than murdering one another, they became partners. The visitor gradually lost its autonomy and changed into something else. Once a free-living bacterium, it evolved into the mitochondrion, which is still found inside almost all complex cells today.
Under a microscope, mitochondria in contemporary cells appear normal, bean-shaped, and silent. However, their presence seems dubious, akin to a fossil concealed in plain sight. They silently preserve proof of their ancient origin by carrying their own DNA apart from the rest of the cell.
Everything changed as a result of this collaboration.
Cells suddenly had access to a lot more power than they had previously thanks to internal energy generators. They were able to store more genetic information, develop internal structures, and enlarge thanks to that energy. Once unachievable, complexity is now attainable. Observing this through genetic analysis and fossil records, one gets the impression that evolution stumbled into complexity rather than progressively ascending toward it.
The tale has become even more bizarre in light of recent findings regarding Asgard archaea. Named after Norse mythological realms, these microbes were found in ocean sediments and seem to be closely related to the host cells that were involved in that ancient merger. It was once thought by scientists that these organisms only existed in oxygen-free environments, so it was challenging to explain their cooperation with bacteria that use oxygen.
However, the more recent results point to an unexpected conclusion.
There are some Asgard archaea that seem at ease with oxygen. They had not fled from it. And they were getting used to it. This insight completely alters the scene. The likelihood of an encounter between two incompatible organisms could have been increased if they had shared environments rather than meeting by chance.
The story starts to come to life as you stand close to coastal sampling locations and watch researchers drop instruments into the water. Similar circumstances might have existed when that initial merger took place billions of years ago based on the mud, the salt, and the subtle decay odor.
Scientists are still working to replicate this event in controlled experiments. In one instance, scientists used specialized equipment to introduce bacteria into fungal cells while closely monitoring the development or breakdown of cooperation. The microorganisms occasionally adjusted to one another. They didn’t always.
Those experiments have an air of vulnerability.
Most partnerships don’t work out.
The original event is all the more remarkable because of that fact. Humans and other complex life may have existed as a result of a partnership that nearly failed. It wasn’t assured. It wasn’t a given.
It was unlikely.
Observing people going about their daily lives on any congested street makes it hard to overlook the subtle irony. Trillions of mitochondria are carried by every person passing by; they are all descended from that ancient microbial visitor. Without acknowledgment, these structures keep up their operations, generating energy and supporting life.
It’s common to refer to evolution as competition. The fittest survive. However, this tale implies something different.
The cooperative’s survival.
