A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about how humans used to eat. It gives a broad overview of the past eating habits of humans. It does not use any pronouns or definite articles that would need prior context. Why other lines are not chosen? Paragraph 2 starts with 'Then', which is a conjunction. Paragraph 3 starts with 'Since then', which also needs prior context. Paragraph 4 starts with 'And', which is also a conjunction. All these sentences cannot stand alone without context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about cooking. It shows the final point about what humans learned after cooking meat.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it shows a logical order of events. After humans ate raw meat, they learned to light a fire.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shows a cause and effect relationship. Learning to light a fire allowed humans to start eating cooked meat.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it continues the topic of cooking. After starting to eat cooked meat, humans learned to cook other foods.