Based on the phylogenetic tree in Figure 28.2, which of the following statements is correct?
a. The most recent common ancestor of Excavata is older than that of SAR.
b. The most recent common ancestor of SAR is older than that of Unikonta.
c. The most basal (first to diverge) eukaryotic supergroup cannot be determined.
d. Excavata is the most basal eukaryotic supergroup.

Verified step by step guidance

1. Understand the concept of a phylogenetic tree: A phylogenetic tree is a diagram that represents evolutionary relationships among various biological species based upon similarities and differences in their physical or genetic characteristics.
2. Identify the key terms: 'Most recent common ancestor' refers to the last shared ancestor between two groups. 'Basal' refers to the earliest diverging lineage in a phylogenetic tree.
3. Examine the phylogenetic tree in Figure 28.2: Look for the branching points that indicate common ancestors and note the order of divergence among the eukaryotic supergroups.
4. Compare the branching points: Determine which supergroup has the oldest common ancestor by comparing the positions of the branching points for Excavata, SAR, and Unikonta.
5. Assess the basal lineage: Evaluate the tree to see if there is a clear indication of which eukaryotic supergroup diverged first, or if the tree does not provide enough information to determine the most basal group.

Key Concepts

Phylogenetic Tree

A phylogenetic tree is a diagram that represents evolutionary relationships among various biological species based on similarities and differences in their physical or genetic characteristics. It shows how species are related through common ancestors, with branches indicating divergence points. Understanding the tree's structure is crucial for interpreting evolutionary timelines and ancestral connections.

Eukaryotic Supergroups

Eukaryotic supergroups are major classifications within the domain Eukarya, which includes organisms with complex cells containing a nucleus. The main supergroups are Excavata, SAR (Stramenopiles, Alveolates, and Rhizaria), Archaeplastida, and Unikonta. Each supergroup represents a distinct lineage, and understanding their evolutionary relationships helps in determining the order of divergence among them.

Most Recent Common Ancestor

The most recent common ancestor (MRCA) refers to the last shared ancestor between two or more species or groups before they diverged into separate lineages. In phylogenetic analysis, identifying the MRCA helps in understanding the evolutionary timeline and the relative age of different groups. It is a key concept for determining which group diverged first or which ancestor is older.