Do you want to know why Ellen Ripley is such a well-written female heroine? It's not because they were making a film and said, "We need a strong female lead," or because some focus group asked for a sci-fi horror flick featuring a strong, independent woman. It wasn't even a clever take on the "last girl standing" trope from every slasher film of the era. No, it was simply that Dan O'Bannon, the person who wrote the script, never even considered Ellen Ripley's sex. He originally wrote it for a man, but when it came time to cast the lead role, they accepted auditions from anyone—man, woman, young, old, black, white—whoever was the best fit for the role.
At the end of the casting process, the role was offered to Meryl Streep! However, she had just lost a close friend and was too much in grief to accept the job. So the role went to the wonderful Sigourney Weaver, who made it one of the most iconic roles in cinema history.