LUCA is inferred from universal genetic and biochemical features shared across modern cells, representing a population ancestral to all surviving cellular lineages rather than the first life.
The last universal common ancestor (LUCA) is the inferred population from which all living lineages descend, not the first life-form.
Genes and biochemical systems shared across bacteria, archaea and eukaryotes can reflect inheritance from an ancestral population. Horizontal gene transfer and later evolution mean the reconstruction is a model with uncertainty.
Use evidence cautiously:
A conserved translation component in all domains supports common ancestry more strongly than a trait found in only one modern group.
LUCA is not necessarily the first cell and need not have possessed every feature seen in modern organisms.
A2.1 is usually tested as a connected argument, not as isolated facts. A strong answer builds a chain: early Earth conditions made abiotic organic synthesis plausible; origin models require small molecules, polymers, self-replication, and membranes; experiments and models support parts of the chain; LUCA, dating, and vents provide later evidence and constraints. The highest-scoring habit is to say “supports” when evidence supports, and avoid saying “proves” when it does not.