By growing an unusual tentacled microbe in the lab, microbiologists may have taken a big step toward resolving the earliest branches on the tree of life and unraveling one of its great mysteries: how the complex cells that make up the human body—and all plants, animals, and many single-celled organisms—first came to be. Such microbes, called Asgard archaea, have previously been cultured—once—but the advance reported today in Nature marks the first time they’ve been grown in high enough concentrations to study their innards in detail.
The resulting electron microscopy images reveal complex internal structures suggestive of those in our own cells, adding support to the still controversial idea that ancient Asgard-like microbes may have been the key ancestor of complex cells. The microbe, from 15-centimeter-deep mud in a canal in an estuary in Slovenia, possesses a complex cytoskeleton made of the protein actin, suggesting this structure arose in archaea before becoming an integral part of plant and animal cells. Those findings add to recent work showing Asgard archaea possess genes once thought to exist only in more complex organisms—another indication they may be an important evolutionary precursor.
“This paper is beautiful,” says Buzz Baum, an evolutionary cell biologist at the Medical Research Council’s Laboratory of Molecular Biology. “The images are stunning.” Baum suspects many researchers will want to study the newly cultured microbes.
Considered a third domain of life by most scientists, archaea are distinct from bacteria and eukaryotes, the evolutionary branch that includes humans. Still, archaea and bacteria bear some key similarities—typically neither has core eukaryotic features such as mitochondria, cells’ internal powerhouses, or DNA encased inside a nucleus, for example. Although many researchers think early eukaryotic cells arose after an archaeon engulfed a bacterium that became the mitochondria, they have struggled to figure out how other features of eukaryotes, such as their many internal membranes and organelles, evolved. “Until recently, life’s journey towards complexity was a blur” says Masaru Nobu, a microbiologist at the National Institute of Advanced Industrial Science and Technology. [Continue reading…]