Which one of the following phylogenetic trees illustrates the relationship between Archaea, Bacteria and Eukarya? The figure shows four candidate cladograms, labelled P, Q, R and S, each grouping the three domains of life in a different branching pattern.
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Molecular evidence groups Archaea and Eukarya as sister lineages, with Bacteria branching off earliest.
This question checks whether we remember which two of the three domains of life, Bacteria, Archaea, and Eukarya, are more closely related to each other. The figure gives four trees (P, Q, R, S) that each pair up two domains as sisters and leave the third as the outgroup. Let's work out the right pairing first, then match it to a tree.
What does the molecular evidence say: Comparisons of ribosomal RNA and of the core machinery cells use to copy DNA and read genes (including proteins similar to histones) show that Archaea and Eukarya share more features with each other than either shares with Bacteria. This tells us Bacteria was the first lineage to branch away from the common ancestor of all life, leaving Archaea and Eukarya to share a more recent common ancestor.
Tree P: Groups Archaea with Eukarya as the closer pair, with Bacteria as the outgroup. This matches the molecular evidence exactly.
Tree Q: Groups Bacteria with Eukarya instead, leaving Archaea as the outgroup. This puts the wrong domain next to Eukarya.
Tree R: Also pairs Bacteria with Eukarya, just drawn with a different branch style than Q. It carries the same wrong grouping.
Tree S: Groups Bacteria with Archaea, leaving Eukarya as the outgroup. This misses the Archaea-Eukarya link entirely.
Only one of the four trees, P, has Archaea and Eukarya joining first and Bacteria branching off at the base, which is what current evidence on the three domains supports.
Let's summarize:
Bacteria is the earliest-branching domain.
Archaea and Eukarya are sister lineages, more closely related to each other than to Bacteria.
