Astro 10-Lecture 8:Astrobiology: Life in the universe

What is life?

How did life arise on Earth?

How likely is life on other planets?

How likely is intelligent life on other planets?

First some biology.What is life, anyway?

Properties of life on Earth– Composed of Cells

• Not necessarily a universal property of life

• Some non-living things have cell like structures

– Uses energy• Many things that are not life also use energy

– Self Replicating• Based upon a stored pattern or template (DNA)

• Some non-living chemicals can be considered to be self replicating.

– Capable of responding to the environment.• There are non-living things that also respond to the environment.

– Maintains homeostasis (internal balance)• Also present in some non-living things (i.e. climate on the Earth).

– Capable of evolution due to imperfect replication.

Evolution: How life changes over time.

• The details of how an organism functions is controlled by stored information (the genetic code).

• If replication of the genetic code is imperfect, the code can change over time. – Changes (mutations) can be beneficial, neutral or detrimental.

– Detrimental and neutral changes are more common than beneficial ones.

– If a change increases survival or reproduction rates, it is more likely to be passed on to offspring.

– If a change reduces survival or reproduction rates, it is less likely to be passed on to offspring.

– Demo

DNA: The genetic code

Long strands of amino acid pairs:

• Cytosine <> Guanine

• Adenine <> Thymine

Arranged into triplets called codons. Each combination codes for an amino acid.

• These are instructions on how to make proteins.

• 43=64 possible combinations

• 22 amino acids+start+stop

• some have multiple codes

• Large sections of DNA are unused.

DNA: The genetic code

DNA’s double structure allows replication.

Replication isn’t perfect.

Mutations: The Good, the Bad and the Indifferent

Mutations can be hard to judge…

• You probably have about 10 unique ones. Three of them are likely to be harmful (not necessarily to you) ones that will be removed in later generations. The rest are likely to be neutral or beneficial.

• A mutation in “junk DNA” is probably neutral unless it causes that area to be used to make proteins.

• Humans can’t make Vitamin C because one of our ancestors (probably a fruit eater) had a mutation. A neutral mutation for that species, a detriment to us.

• One mutation that enables our brains to be so large may cause us to be more susceptible to disease that most other animals.

• Sickle Cell Anemia mutation confers resistance to malaria if you’ve got one copy of it, causes disease if you’ve got two copies.

Selection: The other half of evolution

• In order for an organism’s genes to survive, it needs to reproduce.

• Genes get removed from the gene pool if an organism

• dies before reproducing

• cannot produce viable offspring

• cannot attract a sexual partner

Both mutation and selection cause the genetic code of isolated populations to become more different. Eventually they can become different enough that they become different species.

When did life start on Earth?

• The earliest definite fossils are about 3.5 Gy old. (Bacterial mats)

• There are chemical suggestions of life as early as 4.2 Gya, but these are the subject of argument.

Abiogenesis: How did life start on Earth?

• Carbon compounds (amino acids, alcohols, formaldehyde, etc.) have been found in star forming regions.

• They are also present in meteors, asteroids and comets.

• Miller and Urey showed that amino acids could be formed from UV or electrical discharge in the atmosphere of the early Earth.

• So far nobody has managed to create self replicating molecules from scratch.

– Earth had millions of years and a lot more organic molecules to do the job.

Theories of abiogenesis.

• Polymerization of long organic molecules on clay?

• Concentration of organic molecules in cracks in rock?– Either results in “chemical evolution.”

• Lot of random chemical reactions lead to formation of many types of molecule

• The most stable resulting molecules last the longest and are able to react with other molecules to produce larger daughter molecules.

• The most stable daughter molecules last the longest and are able to react to form longer molecules.

• Eventually one of the daughter molecules was able to replicate itself.

• Panspermia: Life arrived here from interstellar space.– Doesn’t really solve the problem, just changes its location.

Timeline of life on Earth

• 4.6 Gya Earth Forms

• 4.5 Gya The moon forms after impact of a mars sized body

• 4.2 Gya-4.0 Gya Crust solidifies, oceans form.• 4.2-3.5 Gya First life forms bacteria

(likely got energy from chemosynthesis and/or photochemosynthesis)

• ~2.8 Gya Oxygen releasing photosynthesis(oxygen was toxic to most life

forms) • ~2.7 Gya First Eukaryotes

(Cells with a nucleus)

Chemosynthesis: Energy from chemical reactions used to build sugars

2H2S + 3O2 2SO2 + 2H2O + energy

CH4+ 2O2 2H2O+CO2+energy6H2O+6CO2+energy C6H12O6 +6O2

Timeline of life on Earth

• ~2.5 Gya Continents begin to form-carbonate silicate cycle begins to function

Oxygen accumulation reaches critical level -oceans rust, iron oxide falls to the bottom

-ozone layer forms-most cells incapable of detoxifying oxygen die

out Eukaryotes capable of oxygen metabolism proliferate

• ~1.7 Gya First Multicellular Organisms

• ~670 Mya First nervous systemsFirst land plants

Timeline of life on Earth

• ~530 Mya Cambrian explosion- A sudden increase in organism complexity- All of the modern body types evolve in a

sudden burst of evolutionAnnelids (worms)Arthropods (insects, crustaceans)Mollusks (snails, clams, squid)Chordates (precursors to vertebrates)

• ~430 Mya Fishes evolve

• ~350 Mya Amphibians

• ~300 Mya Reptiles, Mammal like reptiles

• ~230 Mya Dinosaurs

• ~200 Mya Mammals

• ~150 Mya Birds

Lessons from the timeline

• In a rough sense, we can estimate how difficult a step in our evolution is by how long it took to arise.– Yahtzee example. You might get a 6 on the first roll, but you’ll

probably need to roll a lot of times to get Yahtzee (5 of a kind).

– From formation of oceans to life took between 0 and 0.7 Gyr

– From the origin of life to complex multicellular life took 3 to 3.7 Gyr

– From complex multicellular life to intelligent life took 0.5 Gyr

• From our sample of one it appears that life is easy to get started, but that complex life is a very difficult stage to reach. Once you get complex life, intelligent life seems to be a fairly easy step.

Life on other planets

• How likely is life on other planets?– According to our previous slide, life is easy to get started,

so life should be on most every “earthlike” planet.

– Of course we only have one example. Earth might not be a typical “earthlike” planet.

• What is an “earthlike” planet? How do we define that? What are the conditions required for life.

Requirements for “earthlike” life.

• A planet or moon (environments like ours require it)

• Hydrogen, carbon, nitrogen, oxygen, various metals.• Liquid water (cells like ours need it to function)• An energy source (hydrothermal or solar)

• Maybe all that is really necessary for any type of life are #2, #4, and some sort of liquid.

Life elsewhere in the solar system

• Do these conditions exist anywhere else in the solar system?– Venus and Mars, no liquid water (or any other liquid).

– Mars probably had liquid water in the past.• Some claim Martian meteorites contain signs of life. Others claim these are

“microfossils” are made by non-living processes

– Giant planets? No solid surface. Maybe life floating in the atmosphere, or life in liquid hydrogen ocean? We may never know.

– Moons?

• Europa has liquid water underneath the ice, and hydrothermal energy source.

• Titan has an atmosphere of methane and ammonia. Organic molecule smog. Liquid methane on the surface? Energy source? Very cold...

Life in other solar systems

• What about other solar systems?

• Are there other planets?– Yes, we’ve found over a hundred using Doppler shift

measurements showing planets tug on other stars.

– We can only detect Jupiters at this point. Maybe we can detect Earths in 25 years or so.

Habitable Zones

For stars of different types, there is a range of distances in which an Earthlike planet could support life– Inner edge: runaway

greenhouse

– Outer edge: runaway freeze

The Drake Equation

N=Ns fs fp ne fl fi fc FL

N = number of communicating civilizations in our Galaxy right now

Ns= number of stars in the galaxy (2x1011)

fs = fraction of stars suitable for life (0.05-0.3) [1x1010 - 6x1011]

fp = fraction of suitable stars with planets (~0.1) [1x109 - 6x1010]

ne = average number of habitable planets or moons per solar system (0.01-0.5)

[1x107 - 3x109]

fl = fraction on which life develops (0.01 - 1.0) [1x105 - 3x109]

fi = fraction on which intelligence develops (0.001-0.5) [100 - 1.5x109]

fc = fraction which try to communicate (~0.5) [50 - 7.5x108]

FL= fraction of the star’s life during which communicative civilization lasts (1x10-8 - 1x10-3)

5x10-7 <N< 750,000