NOVA | scienceNOW | Let's Make a Microbe!

What is life, anyway? It's surprisingly hard to define, even for biologists. Yet there is some consensus on the key traits that distinguish a living creature from non-living stuff. Get a sense of what it means to be alive by meeting our VERY simple cartoon microbe. You can also glimpse some of the progress being made in actual efforts to create synthetic life.—Susan K. Lewis

Trait #1: Have a Container

Your life-form needs a physical or chemical barrier to make it distinct from its environment. This barrier keeps the "chemistry of life" in a certain spot, rather than allowing it to disperse. It traps essential things—like genes—inside but permits other materials needed for growth and reproduction to enter. The barrier also gives your microbe its own identity, which can persist through time.

Synthetic Life Report: Have Containers Been Made?
Most definitely. The oily membranes of living cells today are made of molecules called lipids. In the early 1960s, researchers found that lipids extracted from egg yolks, when dropped in water, spontaneously assemble into cell membrane-like bubbles. Such bubbles, called liposomes, likely were present on the early Earth and could have engulfed other molecules to form the first primitive life. In the past decade, scientists have coaxed liposomes into trapping genetic material in a way that might have occurred in ancient tide pools. They also have shown that, with the addition of more lipids, the containers can grow, and when squeezed by physical pressure, the containers divide in a crude form of self-replication.

Trait #2: Have an "Engine"

In a simple sense, your microbe needs to be able to "eat"—to metabolize energy from the environment in order to grow, repair itself, and reproduce. The engine shown here is pure fantasy. Living cells today are powered in a variety of ways. Plants use photosynthesis, relying on sunlight to make sugars. The cells in your body have mitochondria that convert the energy in food into ATP, a molecule that fuels most of the cell's functions.

Synthetic Life Report: Have "Engines" Been Made?
Yes. Researchers have crafted ways for synthetic cells to harness energy. In one line of work, scientists embedded smelly substances called polycyclic aromatic hydrocarbons (PAHs) into the walls of liposomes. The PAHs enabled the quasi-cells to capture energy from sunlight, much like chlorophyll does in plants. Once again, the research may hold clues to Earth's most primitive organisms. Asteroids likely carried PAHs to Earth four billion years ago.

Trait #3: Have "Instructions" and a "Scribe"

All life today needs its own instruction book. This genetic information guides the initial assembly of the organism as well as directs how it will grow and thrive. It's also critical that the organism have a way to copy these instructions to pass along to future generations. For most life today, including human beings, the basic instruction system is pretty simple: information is encoded in DNA in the sequence of four chemicals known by the letters A, T, G, and C. But then it gets more complicated. The DNA is transcribed into another genetic material called messenger RNA in order to make proteins—the workers and building blocks of the cell.

Synthetic Life Report: Have "Instructions" and a "Scribe" Been Made?
Almost. Many biologists think that the first life on Earth had instructions encoded not in DNA but in a primitive form of RNA. In this so-called "RNA World," RNA by itself could have played all the roles that DNA, RNA, and proteins do in cells today. Researchers are now attempting to make a synthetic life-form that relies on RNA. In proof-of-concept experiments, they have shown that RNA can act both as the "instructions" and the "scribe." So far, this RNA system, called a ribozyme, has only copied a short stretch of itself, but further work could lead to systems that completely self-replicate.

Trait #4: Have the Ability to Evolve

If it couldn't adapt to changing environments, life on Earth likely would have petered out early on. That's why most lists of the key traits for life include the capacity to evolve. Like all living cells, our mock microbe had offspring that were not exact clones. As it reproduced, mistakes were made in the copying of its genetic code. Some of the offspring have traits that make them fit to survive and reproduce in their new home, but others have reached an evolutionary dead end.

Synthetic Life Report: Have Evolving Systems Been Made?
Yes and no. Starting with the pioneering work of Nobel Laureate Manfred Eigen in the 1970s, a number of research groups indeed have constructed evolving molecular systems. The trick remains, though, to create an evolving system that has all the essential properties of life—a "container," an "engine," "instructions," and a "scribe"— inherently linked together. If scientists can make such a system—capable of replicating itself with genetic changes that are subject to Darwinian evolution—it might be hailed as the first synthetic life-form.

Okay, It's Not So Easy

If it were really straightforward to make life in a beaker, we would be surrounded by synthetic creatures. Since Stanley Miller and Harold Urey conducted their famous experiment making amino acids from a soup of simpler chemicals in the early 1950s, hundreds of scientists have tried, essentially, to make life in a lab. The work has shed light on the steps primitive life may have taken on Earth four billion years ago. It has also deepened our understanding of how extraordinarily complex most organisms are today. Researchers remain ever optimistic, but the first truly synthetic life may be decades away—if it can be made at all.