Research School of Earth Sciences
We are made of star dust
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
One way of answering the question ‘What is life?’ is to look at the ingredients of life on Earth. In terms of chemical elements, oxygen, carbon, hydrogen and nitrogen, make up 96.8 ± 0.1% of the mass of life (based on humans and bacteria). Phosphorus and sulfur together make up 1.0 ± 0.3%. The remaining 2.2 ± 0.2% is dominated by potassium, sodium, calcium, magnesium and chlorine, while 0.03 ± 0.01% is attributed to trace elements such as iron, copper and zinc.
All atoms that are or have been part of living matter on Earth have either been produced during big bang nucleosynthesis or in different processes of the stellar nucleosynthetic pathways that take place in stars. Around ~4.5 billion years ago our Sun was formed out of a collapsing molecular cloud that was polluted by earlier stellar processes and a proto-planetary disk that was made of the remaining dust gave rise to terrestrial planets like the Earth.
While the elemental composition of planets reflects to a large extent the composition of the Sun, relative to the Sun, the Earth is depleted in the most volatile elements hydrogen, helium and the noble gases. These elements were swept away by the solar wind from the region of the solar nebula where rocky planets like Earth formed. However, later input of volatile elements from chondritic material, comets and other objects from the outer solar system led to a surface crust on the Earth which exhibits elemental abundances more like the Sun depleted in volatile elements than the bulk Earth.
Life does not reside in the mantle or the core of the Earth and so its elemental abundances are more reflective of abundances in the crust (specifically the biosphere) than abundances in the bulk Earth.
Since, the abundance of most elements in life forms and their environments on Earth follow cosmic abundances (as represented by the Sun), perhaps extraterrestrial life on Mars or moons of Jupiter and Saturn or perhaps extra-solar planetary systems, will also exhibit elemental abundances similar to those found in life on Earth.