Early cell culture studies mainly used cells isolated from a frog, where incubation was not required since frogs are warmblooded animals. However, as investigations became faster and more complex, a model system which more closely resembled human systems was required. The first choice was rodents, which produced continuous cell lines and the transgenics were easier to achieve. Human cells, however, were more difficult to culture. This difficulty was overcome initially, with many thanks to Henrietta Lacks, who was descended from the slaves and lived on the plantation of her ancestors. Henrietta had a very aggressive tumor all over her body. The cells isolated from her cervical tumor samples propagated readily in culture systems,
called HeLa cells, and they are still being used today, some 60 odd years later.