Thales (624 -547 B.C.): He is the first person known to have sought natural rather than supernatural explanations for the world. Used observation and experience as basis for theories. Believed that rational enquiry AND observation leads to knowledge of the universe. Taught that the Earth and the stars were made of the same stuff.
Anaximander (611 - 546 B.C.): Developed a primitive cosmological model and a theory of evolution of sorts. Main contribution was using deduction within science: one generates corollaries from an initial hypothesis. Introduced the concept of mathematical modeling expanded upon later by Pythagoras (580 - 500 B.C.). Anaximander asserted that worlds were created and destroyed.
Anaxagoras (500 - 428 B.C.): In his attempts to understand the cause of eclipses and lunar phases he speculated that the Moon was a body like the Earth. This assumption implies that the Moon "shines" via reflected sunlight. This, in turn, allowed him to correctly explain lunar eclipses - the Earth's shadow falling on the Moon; and for solar eclipses - the Moon blocking the light from the Sun. This also led him to assert that the Moon, too, was inhabited. Indeed, Anaxagoras proposed an early version of 'panspermia' -- the theory that "seeds of life" are dispersed throughout the universe.
Aristotle (384 - 322 B.C.): Argued for a spherical Earth but had some ideas that we can now show to be incorrect, but they were based in rational, observation based science e.g. the Earth must be stationary else rushing winds, stellar parallax, objects dropped from a tower would land away from the base. Also believed that the heavens were unchanging/perfect in nature and made of a fifth element (the quintessence).
"The male has more teeth than the female in mankind, and sheep and goats, and swine...Those persons with the greatest number of teeth are the longest lived; those who have them widely separated, smaller and more scattered, are generally more short-lived."
Aristarchus (310 - 230 B.C.): Discovered methods for calculating the relative sizes of and distances to the Sun and Moon via observations. (The quarters of the moon will not be at exactly 180 degrees separation. ) Developed the first heliocentric model and explained the lack of stellar parallax as a question of the great distance to the nearest stars. Unfortunately, he did not have the following of Aristotle.
Ptolemy (100 - 200 A.D): Geocentric model which fit extremely precise (for the time) the measurements of planetary positions as projected onto the celestial sphere. The model was quite ad hoc as it was based on the near religious (but practically mathematical) restriction of circular motion.
From the medieval to renaissance
The medieval Church : Did not agree with the scientific methods of Aristotle, but found itself in full agreement with his cosmology regarding the special place held by the Earth at the centre of it all.
Nicholaus Copernicus (1473-1543): Reintroduced the idea
of heliocentrism – a break from the 2,000 year reign of Aristotle.
This is broadly interpreted as the Copernican principle -- we do not hold
a special place in the cosmos..
Tycho Brahe (1546-1601): Provided the precise and abundant
observations needed to determine the orbits of the planets.
Johannes Kepler (1571-1630): Carried out the analysis
of Tycho’s data and discovered the empirical laws for planetary motion.
Galileo Galilei (1564-1642): Provided confirmation of
the predictions of the heliocentric model. First to use the telescope
astronomically.
Isaac Newton (1642- 1727): Discovered the basic
physical laws which give rise to Kepler’s laws. Ushered in
the mechanist/determinist view of the universe. His "Universal Law
of Gravitation" asserted that the same law(s?) governed both heaven and
earth. This extension of the Copernican principle puts all later
biological discoveries in a different light.
Christiaan Huygens (1629-1695): Took the Copernican/mechanist
logic to its natural conclusion and argued for the existence of life on
all the planets of the solar system.
Modern
Gregor Mendel (1822-1884): dicoverer of genetics
Charles Darwin (1809-1882): Theory of evolution
Pasteur/Pouchet spontaneous generation debate (1860): Pasteur "proved" that only life begat life. Unfortunately, this was used as evidence of the need for divine intervention in the origin of life on Earth.
Challenger (1876): The failed search for evidence of terrestrial life's origins.
Einstein/Hubble/Penzias and Wilson big bang model (1916-1964): Showed that the universe is of a finite age and perhaps finite in size.
Watson and Crick (1953): Discovery of structure of DNA
Miller and Urey experiment (1953): Perform first experiment intended to replicate conditions on early Earth to see if life can be built from mere chemistry (ie abiotically). Oro, Fox, Cech (see below) and others have extended this type of study. The full picture remains beyond our grasp.
Sputnik (1957): The launch of the first of the first spacecraft (and the space age) by the Soviets. Dawn of the age of planetary exploration. More than 40 missions to the planets (all except Pluto) have been undertaken since that time.
Frank Drake (1960): Begins the first scientific search for extraterrestrial intelligence.
Plate tectonics (1960s) Introduces one of the chief difficulties in studying life on Earth -- we have lost the historical record due to plate tectonics. Looking at other planets may give us the information we need to "re-create" conditions on the early Earth.
Ribozymes (early 1980s) Thomas Cech and Sidney Altman
show that RNA has limited catalytic ability as is necessary to creat the
RNA world model of the origin of life.
Theory of KT Event (1980s) Luis Alvarez shows that
extinction of dinosaurs (hence, the rise of mammals) due to cosmic impact.
Canadian Scientist Alan Hildebrand discoveries the crater which proves the
impact of a large body with Earth at time of the K/T extinction. Evolution of intelligent life is based on many contingencies...
First planets found around other sun-like stars (1995) Planets found orbiting stars like the Sun. Astronomers Marcy and Butler in the United States and Mayor and Queloz in Switzerland reported the first detections of Jupiter-like planets orbiting nearby Sun-like stars.
Mars center stage (1996) Claim of evidence for fossil life in Martian meteorite by McKay in the United States. A meteorite blasted from Mars by an impact was purported to contain possible fossil traces of ancient biological activity and life-like structures. While not generally accepted today as proof of ancient life on Mars, this event catalysed modern interest in the question of microbial life on Mars and led directly to the recent onslaught of robotic spacecraft visiting the planet.
Water on Mars (2004) Mars Exploration Rovers find clear evidence of past water on Mars from in situ exploration. This confirms long-standing evidence of a wet past for the Red planet based on earlier observations from orbiting spacecraft.