Seaweed and algae aren’t the only determining factor in how humans have developed, but there could be reasons to be believe they played a significant role.
One of the things that makes humans different from any other species on earth is that our brains are large in proportion to our body mass. Some neuro-chemists believe this is due to a diet that has a sufficient sources of essential omega-3 fatty acids like DHA and EPA. These fatty acids play an important role in brain development and brain function, but are only found in fish, shellfish, seaweed and algae. In fact, fish and shellfish get their omega-3 fatty acids from consuming seaweed and algae. Because of this, many people also believe that humans did not evolve on dry grasslands but rather in damp regions, likely where the land meets the sea.