Dinosaurs, daisies, starfish and humans are all made up of cells containing tiny biological hitchhikers. In all complex life, each cell has an energy-producing organelle that has its own DNA and looks and acts suspiciously like bacteria. In animals, they’re called mitochondria; in plants, they’re called chloroplasts.

How they got there is an open question and one researchers like Brett Baker, a faculty member in The University of Texas at Austin Department of Marine Science, explore. Baker thinks the answer may go something like this:

• About 2 billion years ago, a microbe called an archaeon gobbled up a free-floating bacterium, and it worked well for both parties. The bacterium provided new energy sources to the archaeon, which in turn provided safety and nutrients.

• Over time the two previously free-living microbes spawned a single, hybrid offspring — the first eukaryote.

• All complex, multicellular life on the planet evolved from this new lifeform.

Baker and his colleagues have uncovered evidence that a recently discovered group of microbes, called the Asgard archaea, were the original hosts to give rise to all eukaryotes, including us humans. With support from the Moore-Simons Project, the scientists are now investigating which Asgards are most closely related to eukaryotes and further exploring their physiological interactions and cellular structure.