THE ability of bacteria to cause lethal diseases might be down to their move from sea to land. A new family tree suggests that the move led to modern microbial traits such as thick cell walls, which are key to survival on land.
Prokaryotic bacteria, the oldest and most basic form of cellular life, come in a variety of shapes and sizes evolved over billions of years. Tracing this evolutionary history has proved difficult because of bacteria鈥檚 ability to swap genes easily. To solve this problem, researchers have looked for two sets of genes: one codes for core proteins and the other for ribosomal RNA. These genes are less likely to jump between species, but each method leads to a different tree.
Fabia Battistuzzi of Arizona State University in Tempe and Blair Hedges of Pennsylvania State University in University Park sorted 286 species of bacteria by both their genes and whether they live on water or land. By identifying and eliminating from their analysis repeats in ribosomal genes, they were able to reconcile the core protein and ribosomal data.
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Their findings suggest that two-thirds of prokaryotes descended from a common ancestor that moved from the ocean to land 3.2 billion years ago 鈥� 600 million years before the first fossil evidence (Molecular Biology and Evolution, ).
鈥淭he genetic analysis suggests that two-thirds of prokaryotes are descended from a common ancestor 3.2 billion years old鈥�
Some can form spores or resist radiation, while gram-positive (G+) bacteria, which cause conditions such as pneumonia, have an extra-thick cell wall. Such qualities might have helped them to survive on land. 鈥淎 thick cell wall makes sense to resist drying out,鈥� says Hedges.
These adaptations may have had unexpected bonuses. G+ bacteria lack an outer membrane, which might have made it easier to acquire proteins to infect people, say the team.
The new tree isn鈥檛 likely to close the debate. Radhey Gupta of McMaster University in Hamilton, Canada, says this is an oversimplification that lumps together bacteria too diverse to be a single group.