Life in the Universe
Life in the Universe The only place we know life exists is here on Earth One of humanity s Big Questions is whether it exists elsewhere We can get some clues by considering life s history here on Earth When we do, we can get an idea how likely life as we know it is We should keep in mind that life as we know it may not be the only kind possible But it is the kind that we will be best able to recognize, if it does exist
Artist s depiction of grey
Victoria Beckham You be the judge Artist s depiction of grey
When did life arise on Earth? What do these events tell us about the possibility that life exists elsewhere in the universe?
When did life arise on Earth? The first evidence of life appears ~150 million years after it became possible It is not fossil evidence, but trace chemical evidence The evidence is in the ratio of carbon-12 to carbon-13
Carbon isotope evidence for life In >3.8-billion-year-old rocks like these in Greenland there is a higher than normal ratio of 12 C: 13 C Living things incorporate 12 C more easily than 13 C So the higher ratio is taken as indirect evidence for life
When did life arise on Earth? The oldest fossils of living things date to ~3.5 billion years ago.
Earliest Fossils The oldest fossils of living things date to ~3.5 billion years ago. Fossil stromatolite in 3.5-billionyear-old rock
Earliest Fossils The oldest fossils of living things date to ~3.5 billion years ago. Fossil stromatolite in 3.5-billionyear-old rock This is a living stromatolite Stromatolites are layered structures formed by colonies of bacteria They still exist today, typically in extreme environments like hypersalty lakes and lagoons
Earliest Fossils The oldest fossils of living things date to ~3.5 billion years ago. Fossil stromatolite in 3.5-billionyear-old rock This is a living stromatolite Stromatolites are layered structures formed by colonies of bacteria They still exist today, typically in extreme environments like hypersalty lakes and lagoons Here are some in a lagoon in Australia
When did life arise on Earth? So fossil evidence shows that life certainly existed on Earth by 500 million years after conditions would permit it to survive And chemical evidence suggests it probably existed much earlier But how did it come to be? We don t know
How did life arise on Earth? We do know that all life as we know it has an inside and an outside These are separated by a lipid membrane (along with a cell wall in plants) Vesicles made of lipids are easy to make in the laboratory
How did life arise on Earth? Life as we know it also has a nucleic acid genome containing instructions for building the organism And we know that all of that seems to have been accomplished here on Earth surprisingly quickly So we believe that given similar conditions elsewhere, life will also arise
Necessities for Life As We Know It Nutrient source Energy (starlight, chemicals, heat) Liquid water (hardest to come by) And can only exist if planet is in habitable zone
Are habitable planets likely?
Habitable Planets Definition: A habitable world contains the basic necessities for life as we know it, including liquid water. It does not necessarily have life. There still needs to be sufficient time for life to evolve And certain other requirements as well
Constraints on star systems: 1. Old enough to allow time for origin and evolution (rules out high-mass stars 1%) 2. Need to have stable orbits (might rule out binary/multiple star systems 50%) 3. Size of habitable zone: region in which a planet of the right size could have liquid water on its surface Even so billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.
The Bottom Line We don t yet know how important or negligible these concerns are. The general feeling among most scientists is that microbial life is likely to be common But how common intelligent, technological life like us humans is, is unknown
Exocivilizations If there are other civilizations as advanced as ours. What would they be like? What would the people look like? How would they think? Presumably they would have similar science to ours What would their art be like? What would their philosophy be like? What would their religion be like?