"Why do your visitors want your world? It is so very obvious. It is not you they are interested in particularly. It is the biological resources of your world." 
Earth "is a world that contains immense biological resources, lifegiving resources, resources that are rare and difficult to find in a Greater Community of barren worlds."
"In the Greater Community, biological resources such as blood, plasma, DNA, bone marrow, skin and body organs can be used for their chemical substances. From these, medicines are made. From these, new life forms are bred. These substances are valuable commodities in certain parts of the Greater Community."
"Those who are intervening in your world today are not interested in the resources that you hold most dear. They are interested in biological resources, particularly those resources that represent the building blocks of life—genetic agents, chlorophyll, blood and plasma. These are the things that are valuable and are greatly needed in a universe full of barren worlds." 
- The Allies of Humanity Book Two, Fifth Briefing: What They Want