Before complex life evolved, 1.7 billion years ago, only single-celled bacteria and archaea existed. Bacteria and archaea ― microorganisms together known as prokaryotes ― represent two of the three basic branches of all life. The third branch is eukaryotes, organisms with more complex cells that comprise the higher forms of life, up to humans.
The origin of these complex eukaryotic cells is considered by scientists to be one of the major evolutionary leaps in the history of life on Earth, but how the complex cells evolved from the simple-celled microbes has been something of a mystery.
But a recently discovered class of microorganisms, deemed “marvel microbes” for their unique properties, sheds light on the evolutionary transition from simple to complex cells.
In a paper published this week in the journal Nature, an international team of microbiologists presented its discovery of a microbe that is more closely related to complex life than any other simple microorganism.
“Our new paper tells what our microbial ancestors might have looked like,” Dr. Thijs Ettema, a biologist at Uppsala University in Sweden and the study’s lead author, told The Huffington Post. “We humans are eukaryotes, and our cells are big and complex. In contrast, the cells of bacteria and archaea are tiny and simple. The main question is how the big and complex cells of eukaryotes have evolved from the smaller and simpler prokaryotic cells.”
The findings build on previous work from famed American microbiologist Carl Woese, who showed that eukaryotes evolved from archaea. The two types of organisms shared a common ancestry, but how the eukaryotes evolved into complex life from small, simple cells was unknown.
The new study details the discovery of a new group of archaea, the Asgard archaea, which provide insight on how eukaryotes evolved. Asgard archaea microbes fall into one of four known lineages, named after Norse gods. They’re typically found in environments such as the sea floor, river sediments, lakes and hot springs. What’s unusual about Asgard archaea is that while they are prokaryotes, just like bacteria and archaea, they have several eukaryotic genes in their genome, which suggests we may have evolved from these microbes.
“[Asgard archaea] represent a sister group of eukaryotes in the tree of life,” Etterma said. “In addition to that, their genomes contain a lot of important eukaryotic genes. Altogether, the discovery of these Asgard archaea tells us that eukaryotes, and hence humans as well, share a common ancestry.”
These “microbial cousins,” as he puts it, already contained certain genes that later became important in the process of eukaryotic cells to become as complex as humans are today.
The fact that Asgard archaea share several genes with more complex life forms suggests that their cells may feature something similar to the structures that make eukaryotes complex, only more simplified. To be sure, scientists need to conduct further research, putting their cells under a microscope for the first time.
“These organisms are our closest microbial relatives, and we know next to nothing about them,” Ettema said in a statement. “Current methods allow us to take a first genetic sneak peek. It is really exciting!’’