1.2 The Recipe for Life

summary

Identifying signs of life

There is no single agreed-upon definition of life. Rather, there are many definitions, some of which are incompatible with each other.
Nevertheless, paleobiologists can now analyze the morphological, mineral, and biochemical signatures of the fossil record to understand the evidence for ancient life on Earth.
This is not particularly surprising – ancient life on Earth is part of the same phenomenon as modern Earth life, so we have a working basis from which we can understand it.
The biggest challenge is to identify signs of life on other planets beyond the Solar System, called exoplanets. The chemistry of life on exoplanets may be intrinsically different from life on Earth, which could make it very difficult to recognize alien biospheres.

How do we define life?

The NASA definition of life is a self-sustained chemical system capable of Darwinian evolution.
This definition is broad enough to include many of the specific processes that characterize life on Earth, including growth, reproduction, metabolism, responsiveness to the environment, and homeostasis.
It is important to remember that none of these individual attributes alone defines life. For example, crystals are organized and can grow but are decidedly not alive.

An astronomical perspective

The search for life on exoplanets focuses on the aspect of biology concerning chemical systems. There are observable attributes to such chemical systems that we have seen on Earth, both today and in the past, which are essential to life and which could be detected using astronomy.
Darwinian evolution is particularly interesting in this context not only because it allows life to be adaptive and self-optimizing, but also because we now know that evolution can take place at the molecular level, without even involving cells.
However, evidence of evolution is extremely difficult to obtain using astronomy. Therefore the evolutionary complexity of life plays a secondary role to its basis as a chemical system in the astronomical search for life on distant planets.

The cosmic limits for life

The observable universe is dominated by three major constituents: ordinary (baryonic) matter, dark matter, and dark energy. We know enough about dark matter and dark energy to say that the complexity necessary for life is unlikely to be achievable with these entities.
Within the realm of ordinary matter, hydrogen and helium dominate the universe. Only very recently in the history of the universe has enrichment with other chemical elements taken place.
Hydrogen and helium are woefully inadequate at building chemical structures, including the structures that could perform most of the actions necessary for life.
The periodic table of elements, particularly oxygen, nitrogen, phosphorous, and carbon, is necessary for the emergence of the phenomenon of life. Some elements are found in living systems in large quantities, while others in only trace amounts, but the diversity of atomic building blocks afforded by the complete periodic table is critical to the formation of life.