The greatest evolutionary change enabling the land vertebrates to be completely free from water , was the development of

Options

A. four legs
B. lungs
C. shelled eggs and internal fertilization
D. four chambered heart

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the significant evolutionary change that allowed land vertebrates to live entirely free from water. This implies that the change must enable these organisms to reproduce and develop without the need for aquatic environments.

2. **Identifying Early Vertebrates**: The first vertebrates to transition to land were amphibians. However, amphibians still require water for reproduction, as they lay their eggs in water, and their larvae (tadpoles) are aquatic.

3. **Evolution of Reptiles**: The next major group after amphibians is reptiles. Reptiles represent a significant evolutionary advancement because they can complete their life cycle entirely on land.

4. **Key Adaptations**: The two primary adaptations that allowed reptiles to be free from water are:
- **Amniotic Egg**: This type of egg has a protective shell and membranes that prevent desiccation (drying out), allowing the embryo to develop in a stable environment without water.
- **Internal Fertilization**: This adaptation allows fertilization to occur inside the female's body, eliminating the need for water to facilitate the meeting of sperm and egg.

5. **Evaluating Options**: The options provided in the question need to be evaluated:
- Four legs: This is a feature of many vertebrates but does not specifically relate to being free from water.
- Lungs: While lungs are important for breathing air, they do not directly address the reproductive aspect.
- Amniotic egg and internal fertilization: This is the correct answer, as it directly relates to the ability of reptiles to reproduce without water.
- Poikilothermy: This refers to temperature regulation and is not directly related to the ability to live free from water.

6. **Conclusion**: The correct answer is the development of the amniotic egg and internal fertilization, which allowed reptiles to reproduce and thrive completely away from water.

**Final Answer**: The greatest evolutionary change enabling land vertebrates to be completely free from water was the development of the amniotic egg and internal fertilization.

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