If an extraterrestrial were to visit Earth, the first thing they would likely notice about human beings would be our physical appearance. As they observe us, they would notice that we are bipedal creatures, meaning we walk on two legs. This trait is not as common among all Earthly species, so it might be intriguing to them.
They would also notice our relatively large brains in comparison to our body size, which would suggest a higher level of intelligence compared to many other animals on Earth.
Additionally, they might observe our diverse range of physical appearances due to various ethnicities and genetic variations across the human population. This diversity could be fascinating to them, as it showcases the complexity and adaptability of our species.
Depending on their level of technology and observational capabilities, they might also notice our advanced civilization, with structures like buildings, roads, and various forms of transportation. They may observe how we interact with technology, use tools, and organize ourselves in societies.
The extraterrestrial visitors would likely be curious about our behaviors and interactions, such as communication, social structures, and possibly our emotional expressions. They might study our languages and attempt to understand our ways of communication.
Overall, the first thing an extraterrestrial would notice about human beings would likely be our physical characteristics, intelligence, and the complexity of our civilization and societies.