life in the universe Flashcards
3 recent developments indicate the high possibility of life to exist elsewhere as well:
Life arose quite early in Earth’s history It may form quickly elsewhere as well, provided it has the right conditions
Organic molecules could be formed even with laboratory chemical reactions
strong indicator that Life may appear by naturally occuring chemistry wherever it has the right conditions Microscopic living organisms were discovered to survive in conditions similar to those on at least a few of other planets in our Solar system.
Earth formed
~4.5 b years ago (b.y.a).
Initial period (4.5…3.8 b y.a.), especially in the 1st several m y.: Heavy bombardment
intense flux of many asteroids & comets, including large ones (even planetesimals) Moon formation (30…50 m y. after Solar syst. formed )
At the end of this period: the Late Heavy Bombardment (3.9…3.8 b y.a., for a period of about 20 to 200 m y.)
→ nr. of impacts in the Solar syst. may have increased tremendously.
Most probably because of shifts in the orbits of the giant planets.
Mineral evidence suggests oceans had formed after the first 200 m y.!
Natural chemistry laboratories that could lead to life.
Some impacts possibly vaporised early oceans → no life possible (on the surface; but theoretically possible below it, or in hot hydrothermal vents!)
Living organisms quickly arose after impacts had subsided.
Evidence suggests Life was already thriving prior to 3.85 b y.a !
History of Life on Earth
deduced from study of fossils buried in layers of sedimentary rocks.
The key issue: determine the correct dates at which fossil organisms lived Radiometric dating → used both in geology & for fossil age estimation (but with different isotopes for each purpose)
Old rocks are much rarer! Earth constantly renews itself!
Those old rocks which can be found have usually been transformed
Geological evidence of Life shows more detail in the last ~500 m years: All prior life were microscopic → Finding fossil evidence of microscopic life organisms is extremely difficult
Stromatolites (colonies of microbes) found in very old rocks.
Date back to 3.5 b years ago Ancient organisms became advanced enough to build stromatolites
There is evidence that Life already thrived even 3.5 b years ago!
Life arose as soon as conditions first allowed it
What is Life ?
There is no simple & clear-cut definition of Life! A more practical alternative is to identify the basic unit of Life
CELL = the smallest unit alive. Cells have 8 basic properties → by extension, these properties can also be considered to be inherent resultant qualities of Life.
Life on Earth:
Is based on carbon (C) → the best ability to readily form bonds and many types of molecules/structures with other C atoms & other atoms, giving flexibility to the form and function that biomolecules can take.
Other chemical elements essential for life are: H, O, N (the most abundant, but others may also play crucial roles, even if their amount is not large). CANNOT SURVIVE without water. Oxygen is also essential for the metabolism of most organisms, especially the most complex ones.
key macromolecules necessary/used in living cells & organisms:
Nucleic acids alongside proteins , carbohydrates and lipids are the major types of key macromolecules essential for all known forms of Life on Earth.
DNA & RNA are nucleic acids
Other important macromolecules are ATP, vitamins & enzymes, hormones, and neurotransmitters.
DNA stores all the essential genetic information necessary for a cell, and even for the entire body the key component involved in evolution
Theories for the origin of Life → 2 categories:
De Novo [from nothing] &
Panspermia [from already existing ‘seeds’]
1) De Novo theories
1.1- Organic Soup Theory:
Abiotic chemical interactions within a ‘primordial soup’
Special polymeric molecules, such as RNA, were created, with the ability to reproduce: the beginning of life.
The Miller-Urey experiment (1953): Organic compounds (including a small percentage of amino acids – the building elements of proteins) were obtained from gases (believed to imitate early Earth’s atmosphere) in a closed system which subjected to heat and electrical discharges.
Subsequent experiments showed that RNA & DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere.
Problems with the Miller-Urey experiment:
Early Earth’s atmosphere was different
Lightning was not continuous
Amino acids and other organic compounds may not have been formed in the large amounts which the Miller/Urey experiment produced
1.2- Surface Metabolists Theory (Günter Wächtershäuser, 1988):
Primitive microorganisms called ‘surface metabolists ’ synthesised and polymerised from inorganic compounds on the surface of minerals with a positive surface charge via an autocatalytic process
1.3- Through iron monosulphide bubbles at hydrothermal vents on the seafloor of the primordial Ocean (Russel & Hall, 1997) :
Problems: Relies on very specific conditions which cannot be ascertained =unclear whether it can be verified and proven experimentally
2) Panspermia theories
Life was already present elsewhere in the Universe, and was transported to Earth, most likely via meteorites or by comets (e.g. during the Heavy Bombardment period)
Its feasibility is well supported by some experimental facts
Delicate bio-molecules are known to have been carried and brought on Earth by meteorites.
ISM clouds and even protoplanetary disks contain not only many simple hydrocarbon molecules, but also many types of complex organic molecules and even important biomolecules Key problem: Panspermia does not answer directly how Life itself arose
HOW and WHEN did RNA form? Before or after the DNA?
Since nucleic acids (DNA & RNA) are necessary to build proteins, and proteins are necessary to build nucleic acids, until the early 1980s it was unclear which came first, the nucleic acid or the protein?
This problem was solved when it was discovered that RNA can both store genetic information and cause or catalyze the chemical reactions necessary to copy itself. Nucleic acids (specifically, RNA) came first — and later on, life switched to DNA-based inheritance
HOW and WHEN did ATP form? Before or after the DNA?
ATP = Primary energy source at cellular level for ALL Earth life.
Studying ATP and other ‘molecular fossils’ revealed that they are closely related to nucleic acids ATP was soon synthesized after nucleic acids
In an RNA world, how did DNA appear? ?
RNA plays a central role in the mechanism of protein synthesis.
DNA synthesis and replication actually requires many proteins.
DNA can be considered as a modified form of RNA
Why was DNA selected to replace RNA?
It is more stable and can be repaired more faithfully.
Why was DNA selected to replace RNA?
→ It is more stable and can
be repaired more faithfully.
One hypothesis: viral replication systems achieved the transition from RNA to DNA : DNA & DNA replication proteins originated in viruses, and DNA replication mechanisms have been transferred subsequently from viruses to cells.
The ‘invention’ of DNA required a complex multi-step process, much more complex than previously thought. Interestingly, this complex process was discovered independently more than once.
The origin of homochirality
how only certain orientations of chiral biomolecules have been selected and used in living cells?
Homochirality is essential to the correct functioning of many of contemporary life-forms
Such highly evolved molecules would not have been available from the outset and it is NOT clear how they could have appeared.
An extraterrestrial source has been suggested for homochiral molecules & their discrimination
Life has appeared only ONCE on Earth! WHY?
Unique conditions, which could not be repeated again There is only one basic life-form on Earth All living species did NOT arise from multiple independent first ancestors. .
How/where did the 1st organisms come from?
We may never know for sure.
Living organisms quickly arose after impacts had subsided
Probably the 1st to appear may have had just a very simple inorganic membrane inside which a large RNA molecule performed all (most?) functions/chemical reactions
Very simple micro-organisms appeared initially
The first to appear: prokaryotes → single-celled organisms (bacteria) with a lipid bilayer membrane but do NOT have a nucleus.
The membrane’s role is critical because it provides the barrier that marks the boundaries of a cell (Isolates the interior of the cell & protects it from the outside, also allowing control over the type & flow rate of molecules in/out of the cell)
Next: Archea → differ from bacteria in that they have a different cell membrane material (NOT a bilayer).
Evolution then led to eukaryotes = single-celled organisms with bilipid layer membranes, nuclei and membrane-bound organelles.
Appearance of eukaryotes
subsequent emergence of multicellular organisms
This ultimately led to the evolution of complex, i.e. highly structured and organized organisms, with specialized organs
Micro-organisms can be further categorised based on their metabolism:
Autotrophs → gain energy & synthesize their own organic materials for growth from inorganic compounds (e.g. CO 2 or CH4) and hence may be thought of as ‘producers’.
Chemotrophs (Chemo-autotrophs) → use inorganic chemical reactions.
Phototrophs (Photo-autotrophs) → use sunlight.
Heterotrophs → require organic material for growth (e.g. dead animals, etc.) and may be thought of as ‘consumers’
Life is divided into 3 main branches
(domains = Archaea, Bacteria, and Eucaria) but all share a common ancestor. Cyanobacteria produced O 2 through photosynthesis :
May have started 3.5 b y.a
Chemical reactions with rocks removed O 2
From 2.4…2 b y.a. O 2 started to accumulate in atmosphere
Oxygen reached higher
% (breathable levels) by 700…600 m y.a. → first complex organisms!
Life on land became possible when atmospheric
O 2 accumulated enough to form an ozone (O 3) layer.
O2 =
initially toxic to most organisms living before ~2 b y.a.
Dramatic change in fossil record ~540 m y.a.
Cambrian explosion → sudden increase in animal diversity!
This sudden burst of new life is also called “Darwin’s dilemma” because it appears to contradict the gradual evolution by natural selection
Age of Reptiles
Life forms developed, complexified and continuously changed WHY? → Evolution Early dinosaurs & mammals arose ~225…250 m y.a.
Dinosaurs dominated for over 100 m years.
Died out ~65 m y.a., most probably due to asteroid/comet impact
Paved the way for large mammals Earliest humans appeared only a few m y.a.
Our industry/technology only existed over the last few centuries
Life, as we know it, is carbon-based. Its requirements are:
Liquid water
Source of energy
Abundant oxygen
Atmosphere that is not poisonous
Abundant & varied food sources
Suitable & stable environmental conditions
Fairly narrow temperature range (–40…60 oC) It may seem that these conditions for Life are narrow & specific. But Life does exist in more hostile environments!
