A basal taxon is a taxon or lineage that diverges early on in the lifelong evolution of a common ancestor. If studied on a phylogeny, the taxon has branched off within the first nodes which are furthest away or closest to the root if not branched off from the root itself, while other nodes have appeared in the same time.
This trend of early divergence causes the taxon to share few similarities with more recently evolved species, any convergent evolutionary traits aside. However, a basal taxon will still share orthologous genes with taxa who share a common ancestor with it. This unity means basal taxa can be studied in parallel and equally for shared, similar, or orthologous systems. One example is the study of genes in yeast which are linked to disease when in the human body, an ethical study which allows for pathologic breakthroughs.