Scientific knowledge is always changing. At one time, the kingdom was the highest level of classification. Today, the highest level of classification is the domain. Two of the domains, Bacteria and Archaea both contain microscopic prokaryotic organisms. What would be the best reason for scientists to separate the prokaryotes? Both organisms do not contain organelles. Research showed differences in living environments. It was determined that neither have chloroplasts. There were too many species of prokaryotes to only have 1 domain for them.

To determine the best reason for scientists to separate prokaryotes into two domains, Bacteria and Archaea, let's analyze the provided options: 1. **Both organisms do not contain organelles.** - While it's true that prokaryotes generally lack membrane-bound organelles, this characteristic alone does not justify the separation into two distinct domains. Both Bacteria and Archaea share this trait. 2. **Research showed differences in living environments.** - This option highlights that Bacteria and Archaea often inhabit very different environments. For example, Archaea are known to thrive in extreme conditions (like hot springs and salt lakes), while Bacteria are more commonly found in a variety of environments. This significant ecological distinction supports the separation into different domains. 3. **It was determined that neither have chloroplasts.** - The absence of chloroplasts is not a distinguishing factor for separating these two domains, as not all prokaryotes have chloroplasts, and this characteristic does not reflect the fundamental differences between Bacteria and Archaea. 4. **There were too many species of prokaryotes to only have 1 domain for them.** - While the diversity of prokaryotic species is a factor in classification, the primary reason for the separation into two domains is based on fundamental biological and genetic differences rather than just the number of species. Based on this analysis, the best reason for scientists to separate the prokaryotes into two domains is: **Research showed differences in living environments.** This option reflects the significant ecological and biological differences that justify the classification of Bacteria and Archaea as separate domains.