tan skin, muschdashes, dirty baseball caps and jump fences real well. also swim polluted rivers well too. not above doing hard days labor for $50. Pack em in your pickup and send them to Immigration Services, and they scatter like cockcroaches. Also known to speak spanish fluently.
subspecies: youth are prone to work hard days labor for $1.50 in shoe sweatshops. Squinty eyes. Black, straight hair, cut at ears. both gender look alike. none look older then 20 until they are 80. known to speak chinese and vietnamese
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or... if you're talking E.T.s, totally different story. unicellular most liekly. arriving on comets or space debris. fossils remnants in ancient water deposits on some plants such as saturn and possibly mar's south pole (if nasa would ever do anything right and get a craft to actually land). if they were advanced, they'd have long arms and fingers, smaller legs. small bodies and larger heads (all adapted for what they need in zero-Gs)