The simplest living organisms on earth are ______

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the simplest living organisms on Earth. We need to identify the category of organisms that are considered the simplest.

2. **Identifying Characteristics of Simple Organisms**: The simplest organisms are typically characterized by their cellular structure. We need to consider organisms that have a less complex cellular organization.

3. **Defining Prokaryotic Organisms**: Prokaryotic organisms are those that lack a nucleus and other membrane-bound organelles. This lack of complexity makes them simpler than eukaryotic organisms, which have a defined nucleus and organelles.

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