An alternative to creating novel organisms through the traditional “top-down” approach to synthetic biology involves creating them from the “bottom up” by assembling them from non-living components; the products of this approach are called “protocells".

Most of the best-known work in synthetic biology is “top down” in the sense that it starts with some pre-existing natural living system and then re-engineers it for some desired purpose , perhaps by synthesizing entire genomes. Another approach to engineering novel biological systems works strictly from the “bottom up” in the sense that it attempts to make new simple kinds of minimal chemical cellular life, using as raw ingredients only materials that were never alive.1 These bottom-up creations are often called “protocells” , as we explain below. In the long run, bottom-up and top-down synthetic biology will increasingly blend and become harder to distinguish.2 In the meantime, though, it is important to separate them, not least because their social and ethical profiles differ. The ethical, social, and regulatory challenges raised by top-down synthetic biology have received considerable attention over the past decade. Bottom-up synthetic biology has only just begun to receive similar scrutiny.