2 History of Life, the Solar System, and the Universe

Geologic Time Scales and Chronostratigraphic Units

The history of the Earth, the Solar System, and the Universe is organized into a formal hierarchy of time and rock units. The Geologic Time Scale is divided into several tiers of classification. Chronostratigraphic units, also known as time-rock units, represent all the strata in the world deposited during a particular interval of time; these include, from largest to smallest, the Erathem, System, Series, and Stage. Correspondingly, Geochronologic units refer to the time interval during which a time-rock unit is formed; these are the Era, Period, Epoch, and Age. A boundary between two systems, series, or stages is defined globally by a Geotype or "driving a spike" at a single physical locality to establish a universal reference point. The Precambrian time encompasses roughly $80\%$ of all Earth’s history, spanning from $4.54\,Ga$ (Giga-annum or billion years ago) to $539\,Ma$ (Mega-annum or million years ago). Notably, the Precambrian record contains almost no fossils compared to later eons.

The Divisions of the Phanerozoic Eon

The Phanerozoic Eon, meaning "Visible life," began $539\,Ma$ and continues to the present day. It is divided into three major eras: the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era ("Ancient life") lasted from $539\,Ma$ to $252\,Ma$. Following a major transition, the Mesozoic Era ("Middle life") spanned $252\,Ma$ to $66\,Ma$, often associated with the age of reptiles. The Cenozoic Era ("New life") began $66\,Ma$ and extends to today. The periods within these eras can be remembered using the mnemonic: "Come Over Some Day, Maybe Play Poker, Try Jeff’s Crazy Pizza, Nacho Queso," which stands for Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian (together making the Carboniferous), Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary.

Evolutionary Milestones in Earth History

Life on Earth has progressed through various stages of complexity since the planet's formation approximately $4.54$ billion years ago. Major milestones include:

  • Prokaryotes: Appeared approximately $4000\,Ma$ during the Eoarchaean.

  • Multicellular life: Emerged approximately $1600\,Ma$ during the Mesoproterozoic.

  • Animalia: First appeared between $650\,Ma$ and $600\,Ma$ in the Neoproterozoic.

  • Land plants: Emerged approximately $500\,Ma$ (Palaeozoic).

  • Mammalia: First appeared between $225\,Ma$ and $243\,Ma$, though non-avian dinosaurs dominated until $66\,Ma$.

  • Hominins: Appeared very recently in geologic terms, approximately $2\,Ma$.

The Origin and Expansion of the Universe

The age of the universe is estimated to be $13.8$ billion years. This determination began with observations by Christian Johan Doppler in $1842$, who described the Doppler Effect. This principle states that the wavelength of any propagating energy shifts if the observer changes position relative to the source. The Doppler equation is expressed as: f=f0(v+vrv+vs)f = f_0 \left( \frac{v + v_r}{v + v_s} \right) where $f$ is the perceived frequency, $f_0$ is the emitted frequency, $v$ is the speed of the wave in the medium, $v_r$ is the speed of the receiver relative to the medium (added when moving toward the source), and $v_s$ is the speed of the emitter relative to the medium (subtracted when moving toward the receiver). In the $19$th century, William Parsons (Lord Rosse) used the "Leviathan of Parsonstown," a $72$" telescope, to sketch spiral "nebulae," now recognized as galaxies such as the Whirlpool Galaxy.

Redshift and the Big Bang

The Doppler Effect applies to light as well as sound, though light travels significantly faster (6.7×108 mph6.7 \times 10^{8}\,mph compared to 760–660 mph760\text{--}660\,mph). Light spectra consist of different wavelengths, with blue light possessing shorter wavelengths (440 nm440\,nm) and red light possessing longer wavelengths (700 nm700\,nm). Vesto Slipher observed that distant nebulae were "redshifted," meaning their light shifted toward longer wavelengths. Edwin Hubble later confirmed that many of these objects were galaxies beyond the Milky Way. Because virtually all galaxies are redshifted, they are moving away from us, indicating the universe is constantly expanding. By calculating the path and position of galaxies backward in time based on redshift, scientists concluded that all matter was once concentrated at a single point, resulting in the Big Bang approximately $13.8 \times 10^{9}$ years ago.

Galaxies and the Milky Way

Gravity is the attractive force objects exert on one another, determined by their masses and the distance between their centers. Gravity clusters stars into galaxies. The Milky Way Galaxy contains between $100$ and $400$ billion stars. Our sun is located near the outer edge of one of the Milky Way's spiral arms and completes one orbit around the galactic center every $250$ million years.

Formation of the Solar System and Planets

The solar system originated as a dense cloud of interstellar gas and dust that collapsed into a solar nebula. This nebula formed a spinning disk, where gravity concentrated $99.8\%$ of the system's mass in the center to form the Sun. A planet is defined as an object that orbits a star, is roughly spherical due to its own gravity, and has cleared its "neighborhood" of other objects. Pluto is excluded from this definition as it has not cleared its neighborhood. Planets are classified into three types:

  • Terrestrial planets: Feature a rock mantle surrounding a metal core.

  • Gas-giant planets: Composed primarily of HH and HeHe gas.

  • Ice-giant planets: Composed of HH, HeHe, and large volumes of H2OH_2O, NH3NH_3, and CH4CH_4 ice. Planetary differentiation occurred early in history as planetesimals heated up due to collisions, internal pressure, and radioactive decay. This caused dense materials to sink into a core while less dense materials formed the mantle and crust.

The Earth-Moon System and the Giant Impact Hypothesis

There is a notable density paradox regarding the Earth. The Earth appears denser than predicted by standard models; however, when the Moon is included in the calculations, the Earth-Moon system density reaches equilibrium. This supports the Giant Impact Hypothesis, which suggests the Moon was originally part of the Earth. According to this theory, two large protoplanets collided early in the solar system's history. A massive amount of material was ejected; some of this material accreted to form the Moon, while the rest was re-accreted into the Earth.