Jessie had taken the risk of time travel after accidentally killing her nephew. Unable to live with the result of her stupidity, she had volunteered to travel back in time. The arrival year was dialled up, after that any traveller was on their own. Jessie found herself residing in an animal, but at least she could telepathically link with other travellers.
However as humanity switched to laboratory grown food, most animals species became extinct, so transfering to new hosts became hard. In 2727 the last Zoo was forced to close and slaughter its animals. Animals on earth were history.