First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of early fish. It gives a broad overview of what the text will discuss. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with 'These', which is a pronoun that refers back to something not yet mentioned. Paragraph 3 starts with 'However', which is a conjunction and shows a contrast, so it cannot be first. Paragraph 4 starts with 'The', which assumes prior knowledge of 'jaws', making it not independent. Paragraph 5 also starts with 'The', which is not independent for the same reason.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion. It summarizes the importance of jawed creatures and their advantages, providing closure to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of jawless fish. It gives more information about their current status, showing a logical order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it introduces a contrasting idea. It uses 'However' to show that while most jawless fish are extinct, lampreys still exist.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it continues the discussion about jaws. It logically follows the mention of lampreys by talking about the first jaws.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it discusses the advantages of jawed creatures. It logically follows the information about the first jaws.