Final answer:
Researchers conclude that the hardened chunks of bitumen are part of boats based on archaeological evidence, Mesopotamian records, and the discovery of the Uluburun Shipwreck.
Step-by-step explanation:
Researchers are able to know that the hardened chunks of bitumen are part of a boat based on archaeological evidence and artifacts found in Mesopotamia. Mesopotamian records reference the 'boats of the magan' and some models and drawings depict boats with oars and possibly masts for sailing.
Additionally, the discovery of the Uluburun Shipwreck off the coast of Turkey provided valuable insight into ancient sailing vessels. The ship contained various materials and cargo from different regions, indicating a robust trade network between cultures. All of these findings and references help researchers conclude that the hardened chunks of bitumen are indeed part of boats.