Can bacteria become humans in the future? How could they become human sentient beings?

In the future, bacteria can not only become human beings, but can also learn Buddhism and cultivate the path, and can even attain buddhahood. The key lies in how they become human. To become a being of a particular realm, one's merit must correspond to that of the beings of that realm. Even becoming a deva of the desire realm, a deva of the form realm, or a deva of the formless realm requires one's merit to correspond to that of the beings of those heavenly realms. If one is to be reborn in other buddha-lands in the future, one's merit must also correspond to that of the beings in those buddha-lands; otherwise, one cannot live together with them. Even among human beings, the members of a family must have corresponding merit; otherwise, they cannot become one family, or the family will fall apart, with each member going their own way and living according to their own merit. The same is true of a group: if their merit differs, they cannot come together or work together.

Bacteria are the most lowly, most inferior, tiniest beings in the sentient-being realm, with the least merit. By slowly accumulating merit on human bodies and on the bodies of larger animals—providing services to these beings and sustaining their basic life activities—once their merit accumulates to a certain degree, they will be reborn as somewhat larger animals. As animals help one another and their merit further increases, they will be reborn as still larger animals with greater autonomy and stronger survival capabilities, until finally they are reborn as human beings. If they have good karmic conditions and encounter the Buddhadharma, they will first hold blind faith in the Buddhadharma, and finally come to hold correct faith in it.

All the merit that sentient beings accumulate comes from serving other sentient beings. Therefore, if we wish to possess greater merit, we should no longer be selfish and self-interested, thinking only of our own petty affairs and refusing to benefit others. The fact is that helping others is helping oneself. The merit one cultivates never falls upon others; cultivating merit is never done for the sake of others. Only by trying your best to let others gain benefit can you yourself obtain benefit.