The traditional tree of life (shown above) presents the three domains as distinct, monophyletic lineages. However, other hypotheses propose different views on the relationships among the Archaea, Bacteria, and Eukarya. In particular, the two-domain hypothesis—or eocyte hypothesis—is emerging as a well-supported alternative to the three-domain hypothesis. The eocyte hypothesis, illustrated below, suggests that eukaryotes evolved from eocytes (also known as the Crenarchaeota—a major lineage of the Archaea). Resolving the relationships among these ancient lineages is difficult, but it has profound implications on our understanding of the origin of eukaryotic cells.

Other hypotheses for the tree of life present the Archaea as the ancestors to all other organisms. Sketch a phylogenetic tree that presents Bacteria and Eukarya as more closely related to each other than to Archaea, and that has Archaea as a sister group to Bacteria and Eukarya.

Verified step by step guidance

1. Begin by understanding the traditional three-domain hypothesis, which posits that Bacteria, Archaea, and Eukarya are distinct, monophyletic lineages. This means each domain forms a separate branch on the tree of life.
2. Examine the two-domain hypothesis (eocyte hypothesis) presented in the image. This hypothesis suggests that Eukaryotes evolved from a specific group within Archaea known as Crenarchaeota (eocytes), making Eukaryotes more closely related to Archaea than to Bacteria.
3. To sketch a phylogenetic tree where Bacteria and Eukarya are more closely related to each other than to Archaea, start by drawing a common ancestor at the base of the tree. From this ancestor, draw two branches: one leading to Archaea and the other leading to a common ancestor of Bacteria and Eukarya.
4. Next, from the common ancestor of Bacteria and Eukarya, draw two separate branches: one leading to Bacteria and the other leading to Eukarya. This configuration shows Bacteria and Eukarya as sister groups, more closely related to each other than to Archaea.
5. Label the branches appropriately: the branch leading to Archaea should be labeled 'Archaea', while the branches leading to Bacteria and Eukarya should be labeled 'Bacteria' and 'Eukarya', respectively. This tree structure reflects the hypothesis where Archaea is a sister group to the combined lineage of Bacteria and Eukarya.

Key Concepts

Three-Domain Hypothesis

The Three-Domain Hypothesis classifies all life into three major domains: Bacteria, Archaea, and Eukarya. This model suggests that these domains are distinct and represent separate evolutionary lineages. Bacteria are prokaryotic organisms, while Archaea, also prokaryotic, are more closely related to Eukarya, which includes all eukaryotic organisms. This hypothesis is foundational for understanding the evolutionary relationships among different forms of life.

Eocyte Hypothesis

The Eocyte Hypothesis, or two-domain hypothesis, proposes that Eukarya evolved from a specific group of Archaea known as eocytes or Crenarchaeota. This model suggests a closer evolutionary relationship between Eukarya and Archaea than between Eukarya and Bacteria. It challenges the traditional Three-Domain Hypothesis by suggesting that the evolutionary tree of life may be more complex, with significant implications for our understanding of cellular evolution.

Phylogenetic Tree

A phylogenetic tree is a diagram that represents the evolutionary relationships among various biological species based on their genetic or physical characteristics. In the context of the question, constructing a phylogenetic tree involves illustrating the relationships among Bacteria, Archaea, and Eukarya, highlighting how these groups diverged from common ancestors. Understanding how to read and create these trees is essential for visualizing evolutionary history and the connections between different organisms.