Final answer:
The 'universal' in LUCA emphasizes that all life diverged from a single source with shared genetic and biochemical traits that persist across all organisms, highlighting the single origin of life on Earth.
Step-by-step explanation:
The letter 'U' in "LUCA" stands for 'universal,' signifying the idea that the Last Universal Common Ancestor (LUCA) is the shared ancestor of all current life on Earth. This term underscores the concept that life on our planet diverged from a single common source, making LUCA the focal point of evolutionary biology. The 'universal' aspect is crucial because it implies that the genetic and biochemical traits of LUCA are universally preserved amongst all living organisms, providing strong evidence for a single origin of life. The evolutionary pathways distinct to life after LUCA are likely to have been markedly different prior to this common ancestor. This is demonstrated by the universality of the genetic code and the shared basic metabolic machinery across diverse life forms. In essence, extensive horizontal gene transfer (HGT), which would have been important in early evolution, could only be effective if there was a common genetic language, further reinforcing the idea that there was just one LUCA.