Messenger comets might be why Earth has life, asteroid Ryugu samples suggest 

 Samples of the asteroid Ryugu, returned to Earth by the Hayabusa2 mission, carry evidence that organic molecules important to life were once delivered to Earth by comets. 

https://cdn.mos.cms.futurecdn.net/6JVRRoZ5JXmZ2EuDFwQiPF.png

https://www.space.com/life-earth-messenger-comets-asteroid-samples-ryugu-hayabusa2