Bone marrow transplantation began 20 years ago.是什么意思？

Bone marrow transplantation began 20 years ago.

骨髓移植开始于20年前。

bone

n. 骨头，尸骨，骨质物，有…样的骨头的；vt. 挑鱼刺，剔骨头，剔去…的骨；vi. 苦学，专心致志；

例句
...a small, expensive pocketknife with a bone handle.
昂贵的骨柄小折刀

marrow

n. 骨髓，脊髓，滋养品，〈英〉食用葫芦；

例句
Some films chill you to the marrow of your bones.
有些电影让人毛骨悚然。

transplantation

n. 移植，移植法，移居，移民；

例句
...the dramatic improvements in organ transplantation in recent years...
近几年器官移植方面取得的巨大进步

began

v. 开始( begin的过去式 )，着手，创始，创办；

例句
He began comically ad-libbing a script...
他即兴调侃起了一段剧本。

years

n. 年( year的名词复数 )，年纪，一年的期间，某年级的学生；

例句
His wife was sentenced to seven years imprisonment for aiding and abetting him...
他的妻子作为他的从犯被判入狱7年。

ago

adv. 以前，过去的，之前；

例句
He was killed a few days ago in a skiing accident...
几天前他在一场滑雪事故中丧生。