In my previous blog posts, I’ve written about how marine microbes shape the oceans, how we struggle to grow some in labs, and why they matter more than we often realize. This time, I want to explore something deeper, and how they challenged the very way we understood evolution itself.
When we think about evolution, we might think of a familiar story. Genes pass vertically from parents to offspring. Mutations happen slowly, and over millions of years these small changes add up, leading to species diverging, and life branching into the incredible diversity we see today. This process creates a branching ‘tree of life’, with each split representing a common ancestor and each branch a lineage – plants, animals, microbes, and countless other forms of life all branching in intricate ways. For a long time, this worked well enough, but for microbes, we know they usually don’t like to follow the rules.
As researchers began to sequence more genomes, they started noticing weird patterns, like genes in the same organism often told conflicting stories about their origin. Some genes were present in completely distant, unrelated species but not with their closer relatives. At the heart of this phenomenon is Dal’s very own, Dr. W. Ford Doolittle, an evolutionary biologist who has spent much of his career asking questions about the assumptions that underlie biology. In the 1970s, the Tree of Life was still considered sacred, and the idea that all life could be organized into a clear, branching structure based on vertical inheritance seemed unquestionable. But Dr. Doolittle suspected that the tree metaphor might not be telling the full story.
His most influential insight came in 1999, when he published a paper in Science that would go on to reshape evolutionary biology. He argued that for microbes, evolution isn’t just a vertical process, it’s also horizontal (also called lateral). He included a hand-drawn sketch of what he called a “reticulated tree”, where the branches are interconnected by horizontal lines, showing genes moving not just downward through generations but sideways across species, kind of like the horizontal branches you see in the picture above. This made the classic tree of life more like a web, or a network- messier, but unquestionably truer to nature. However, the branching tree of life remains a powerful model for complex organisms.
So, microbes don’t only pass genes vertically to their offspring, but they also exchange genes laterally – across species boundaries. This process, known as lateral gene transfer (LGT), allows bacteria to acquire new traits. In the microbial world, who your ancestors are still matters, but so do who your neighbors are. It’s also about access: who they share their environment with, and which genes are floating nearby. A bacterium that acquires a gene for salt tolerance, antibiotic resistance, or a new metabolic ability at the right moment might survive an environmental shift that others can’t.
LGT can have very dangerous implications though, like antibiotic resistance. A harmless bacterium could pick up a resistant gene from its environment or another bacterium, and suddenly gain the ability to resist one or more antibiotics, becoming untreatable. In other cases, bacteria could acquire genes that help them break down oil in contaminated waters. This process can occur in several ways. One common method is transformation, where a bacterium picks up free DNA fragments from its surroundings, usually from a donor bacterium that has died. Another method is transduction, where viruses called bacteriophages transfer genes between different hosts. In conjugation, bacteria form a physical bridge to share DNA directly. These processes are often powered by mobile genetic elements, which are like hitchhikers because they can often move within or between genomes – something I’ll talk about in my next post!
One of the most striking discoveries was that even our own genomes aren’t immune to lateral gene transfer! Over time, segments of DNA from bacteria and other organisms have made their way into the genomes of animals, including humans. This creates tangled evolutionary relationships that challenge how we define species. If genomes are more like mosaics assembled from different sources, then what defines an individual organism? These are complex questions that scientists are still working to understand.
In my own research, I do my part by investigating LGT in marine environments, and specifically, how some bacteria have cleverly cheated their way into gaining new traits like nitrogen fixation. It’s exciting to see how microbial evolution works in such unexpected ways. Life is far more interconnected than we once thought, and evolution is trickier than we could have ever imagined. And no, humans don’t transfer genes like bacteria… thankfully.
Image by Michael Grey from Pixabay
