a group of canadian researchers have discovered a set of genes that determine the lifespan of the common nematode. by manipulating the newly discovered genes, the team was abe to increase the lifespan of a nematode fivefold. altering the genes apparently caused the metabolism of the worms to operate at a more leisurely pace. this caused the dna effects thought to bring about aging to accumulate at a reduced rate. of course the cause of aging in humans are more involved than those in nematodes. however, researchers are confident that these discoveries will provide invaluable clues about the aging process and that they will allow science to eventually delay the process.

* nematode : 선충

밑줄 친 they가 가리키는 것은 무엇인지 알고 싶어요....
그리고 they앞에 쓰인 that은 어떤 쓰임으로 쓰였나요 ???

친절한 답변 부탁드리겠습니다.