Ultra-rare merging of two lifeforms sparks exciting evolutionary prediction: 'We just haven't noticed'

There has only been four documented instances of this evolutionary process in two billion years. Dr Tyler Coale thinks it could be happening again right now.

Two lifeforms have merged into one, and incredibly it's just the fourth time such an event has been documented in the last two billion years. The first occurrence of primary endosymbiosis gave rise to complex life, and the second time we got plants, so this new evolutionary discovery has sparked interest around the globe.

Now the UC Santa Cruz researcher who led the study, Dr Tyler Coale, has revealed there are likely other undocumented instances of this life-form-merging process. When asked if his discovery means it’s statistically more likely there will also be future instances of primary endosymbiosis, his answer was “absolutely”.

“There’s a huge diversity of microbes in the oceans, and really everywhere in all environments that are understudied, and just unknown,” Coale told Yahoo News Australia. “So I think it is very likely that there are other cases of this type of organelle evolution that we just haven't noticed yet, that are probably occurring as we speak,” he said.

Filmed using a microscope, a black arrow points to the organelle inside the nitroplast.
A black arrow points to the organelle inside the nitroplast. Source: UC Santa Cruz

Primary endosymbiosis describes the process of a free-living organism being absorbed into the cell of another and then evolving together beyond basic symbiosis.

To get technical for a moment, in the case of the newly discovered nitroplast, a simple prokaryotic cell was engulfed by a more complex eukaryotic cell. Once inside the eukaryotic cell, the prokaryotic cell essentially started to act as a tiny organ — what's known as an organelle.

  • The first time primary endosymbiosis is known to have occurred was around 1.5 to 2 billion years ago and it created mitochondria which gave rise to complex life.

  • The second event took place some time between 1 and 1.5 billion years ago and it was also significant as it created chloroplast and this formed the branch of evolution that became plants.

  • The third event, 100 million years ago, created a type of colour producing cell called a chromatophore.

  • The fourth event also occurred around 100 million years ago and this gave us the nitroplast.

Coale said the first and second instances of primary endosymbiosis resulted in the creation of life as we know it, but the eventual impact of the third and fourth documented events is not yet known.

“The third event is the one that nobody really knows about because it's obscure. And it's basically a small kind of amoeba that has absorbed a cyanobacteria and evolved an organelle,” he said.

“It’s very small. It has not yet diversified into anything like a humanoid, or a major branch on the tree of life, and neither has the nitroplast. But they’re so recent we wouldn’t really expect that.”

Palaeontologist Kyoko Hagino pulling the organism from the ocean.
The organism was pulled from the ocean and then cultured in the lab so it could be studied. Source: Supplied

The reason primary endosymbiosis occurs is to create a "win win" situation for the two lifeforms — one organism can offer something the other does not have.

In the case of the nitroplast, the process of engulfing an cyanobacteria ultimately gave an algae cell, which had previously survived using photosynthesis, the ability to process nitrogen instead.

It's not known why the cyanobacteria was first absorbed by the algae. "The initial establishment of that interaction may have been to give the cyanobacteria shelter or just a nicer environment to live in," Coale said. But eventually its believed the algae lost its ability to photosynthesise, and so its relationship with the cyanobacteria strengthened, resulting in it becoming an organelle. This is the first time a nitrogen-fixing organelle has been documented.

Coale believes the University of California discovery is relevant to modern agriculture as engineers have been trying to replicate the process artificially.

“It could provide a roadmap towards some synthetic nitroplasts,” he said. “There is interest in the engineering of synthetic organelles of different types. And this provides clues into another organelle that’s much more recent than the ancient ones.”

It’s less certain as to whether the organism could be used to further advances in human medicine. “We’ll have to wait and see. I think there’s certainly a lot of interest in how organelles are established, how they're maintained, and how they evolved. I think it’s too early to say what we might learn from it,” he said.

The UC Santa Cruz research team in the lab - Esther Mak, Jonathan Zehr, Kendra Turk-Kubo and Tyler Coale. Right - a gloved hand holding to vials.
The UC Santa Cruz research team: (from left) Esther Mak, Jonathan Zehr, Kendra Turk-Kubo and Tyler Coale.

The nitroplast is understood to be quite common across the world’s oceans. It was first sequenced by Professor Jonathan Zehr at UC Santa Cruz in 1998 using samples taken from the Pacific Ocean, and his colleagues spent years investigating his strange organism which they called UCYN-A.

At the same time, 9,000 km away in Kochi, Japan, palaeontologist Kyoko Hagino was trying to culture the host organism – something that took her over 10 years to accomplish.

It was originally thought UCYN-A was an endosymbiont – any type of organism that lives within the body or cells of another organism – but what’s now been concluded is that the two lifeforms fused and evolved together.

Coale believes there are likely many similar instances along the spectrum from endosymbiont to organelle that are yet to be discovered. "I think it's very likely that in the future we'll discover more examples that are at all different places along that spectrum," he said.

The discovery has been published in the journal Science.

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