Meaning: 'Footprints' are the impressions left by feet on a surface, indicating movement or presence.

Wrong options:

• age: The word 'age' is a noun that refers to a period of time, which does not fit as it does not describe a physical trace left by humans.
• mechanisms: 'Mechanisms' is a plural noun that refers to processes or systems, which is irrelevant in the context of discussing physical traces.
• overthrows: 'Overthrows' is a verb that means to remove from power or authority, which does not fit the context of discussing fossilized remains.
• overlaps: 'Overlaps' is a verb that means to extend over and cover a part of something, which does not relate to the physical evidence of human presence.
• era: 'Era' is a noun that refers to a significant period in history, which does not describe the physical evidence being discussed.

Correct answer: age

Explanation: The word 'age' is a noun that refers to a distinct period in history, which fits contextually as it describes the time frame during which the footprints were made. It aligns grammatically with the structure of the sentence, indicating a specific time period.

Meaning: 'Age' refers to a particular period in history characterized by particular events or developments.

Wrong options:

• mechanisms: 'Mechanisms' is a plural noun that refers to processes or systems, which does not fit the context of discussing a historical period.
• footprints: 'Footprints' is a plural noun that refers to marks left by feet, which does not fit as it does not describe a time period.
• overthrows: 'Overthrows' is a verb that means to remove from power, which does not relate to the concept of time or historical periods.
• overlaps: 'Overlaps' is a verb that means to extend over and cover a part of something, which does not fit the context of a historical period.
• era: 'Era' is a noun that also refers to a significant period in history, but it is not the correct term in this specific context.

Correct answer: overthrows

Explanation: The term 'overthrows' is a verb that indicates the act of rejecting or invalidating a previously held belief or idea. It fits grammatically as it aligns with the subject of the sentence, which discusses the impact of the new research on existing theories about human arrival in North America.

Meaning: 'Overthrows' means to remove or displace from power or authority, in this context, it refers to challenging established beliefs.

Wrong options:

• age: The word 'age' is a noun that refers to a period of time, which does not fit as it does not convey an action related to changing beliefs.
• mechanisms: 'Mechanisms' is a plural noun that refers to processes or systems, which does not fit the context of challenging ideas.
• footprints: 'Footprints' is a noun that refers to physical traces, which does not convey an action and is irrelevant in this context.
• overlaps: 'Overlaps' is a verb that means to extend over and cover a part of something, which does not relate to the act of challenging existing beliefs.
• era: 'Era' is a noun that refers to a significant period in history, which does not fit as it does not convey an action.