The acronym LUCA stands for—Last Universal Common Ancestor. The term is used by astrobiologists and those interested in the evolutionary biology. The geological period of time in which LUCA existed was a point of demarcation from primitive life forms to more-advanced earth-living entities. The importance of LUCA lies in understanding how the three kingdoms of life came from a universal ancestor.
Description from Source: A phylogenetic tree of living things, based on RNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes.
Original Source: NASA Astrobiology Institute Attribution: By MPF [Public domain], via Wikimedia Commons
You might ask, why is LUCA so important? (brief interpretation)
Glancing at the phylogenetic tree one gains a sense that there may be a commonality or root to everything. ( It is not an oversimplification as much as it is a functional, mnemonic device.) So for sake of argument, the center point of the tree may be the point of LUCA—the point where red, black and purple branches form a ‘Y.’ It is at the point where RNA-life may have taken the first steps to ‘current’ DNA/RNA commonality. (It would be obvious—to evolutionary biologists, at least—that gaining an understanding of LUCA is tantamount to taking the next step backward to the origins of life.) The complex machinery in present DNA/RNA is like a ‘black box’ problem—one knows what goes in and what comes out–but the manner (or mechanism) is unclear. LUCA is a point of ‘transcendence,’ it is in essence a step in the evolutionary ladder. (Once the mechanism is discerned, the understanding may be harnessed for the betterment of the human condition.) Current evolutionary paradigms utilize a random mutations as a means by which ‘the paradigm advances.’ However, random mutations may literally take eons of time until the fit survive the next step of evolution.
It may be the case where an understanding of the transcendence of RNA life to DNA/RNA informs the human condition of how to better utilize life for itself and its progeny.
Source for thoughts and further introspection:
Frontiers of Astrobiology, edited by Impey, Lunine and Funes
Cambridge University Press, 2012