One thing that is common to all of life that we know of is the cell. From single-celled organisms like many bacteria to organisms that are giant colonies of cells like fungi and the Portuguese man o’ war to multicellular organisms like us, polar bears, and tardigrades; all of life as we know it relies on cells as the basic unit of a living thing. Even viruses, which we sometimes have a hard time figuring out if we should consider living or not, rely on cells. A virus needs host cells in order to replicate and to evolve.

Cells are so important for life as we know it that one of the big questions to answer about life is when the very first cells arose on Earth. Studies of the DNA in living creatures imply that all life on Earth shares a common ancestor, and we know from our studies of fossilized cells that cells have been around for a very long time. However, the cell membranes of today are probably too complex to have been the first step in forming a cell membrane. For instance, these membranes in modern cells are often composed of two or more layers of molecules and will have lots of enzymes and other proteins stuck within them. This leads some scientists to think the first cell membranes were simpler.

Simple membranes actually form spontaneously in nature, without any help from living things. For instance, certain mixtures of lipids in a solvent like water will self-organize into cell-like structures (called vesicles). It’s possible that these kinds of simple membranes were what made up the first cells for living things on Earth. These first cells would likely then have become the containers for holding the molecules involved in storing genetic information as well as the chemical reactions of metabolism. Actually, in current studies of the origin of life, there is an active debate as to whether the storage and passing on of information for life came first or if the chemical reactions for storing and using energy for life came first. Either way, the first cells would have provided little microenvironments for concentrating and protecting the molecules that life needed for storing information and for catalyzing chemical reactions.

Once simple cells were established, some incredible changes happened in the oceans. Perhaps the most important was the Great Oxidation Event ( GOE ) occurring between 2.5 and 2.3 billion years ago helped created an oxygen rich ocean for life to thrive. Recent research suggests that stromatolites like those found in Shark Bay in Western Australia contributed to the life giving build-up of oceanic oxygen, even before the GOE .

One other interesting place of study in the origin of life research is whether or not there was one origin of life on Earth or possibly many. When we discuss the common ancestor for life as we know it, we call this common ancestor “LUCA ” (for the Last Universal Common Ancestor). However, that doesn’t mean that LUCA was a single organism or even a single type of organism, but was potentially a group of organisms who happened to be the most successful out of a larger group that were present on ancient Earth. There could have been several competing types of life, perhaps all fairly similar, but that what made LUCA special was that it was able to outcompete the other forms of life for resources at some period of time in Earth’s history. In fact, some people have even considered whether there are modern attempts for new origins of life on Earth. What do you think would be a problem if that were to happen? If you guessed that all of the things that live here already would eat that new form of life, you might be right. We think that one thing that stops new types of life from forming on Earth is that there’s already a rich biosphere full of life here.

Looking for life elsewhere may one day truly help us to better understand ourselves and our own biosphere. Will other life out there be cellular life? Will it be something different? We really don’t know yet.