Complex sentence: By comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions; some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate.
Split into simpler sentences: By comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions. Some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate.