Topic 5. Lecture 8. Earth and Fossils.
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So far, we studied past evolution of life using indirect evidence, contained in modern life. However, we also possess a lot of direct evidenceof past life and its evolution, in the form of fossils. Fossils provide a lot of important information, some of which could never be inferred from modern life.
Indirect and direct
evidence of past
Moreover, evolution occurred on Earth, and a lot is known about the past of the Earth itself. This is important, because:
1) We cannot understand fossils without knowledge of the Earth's history,
2) Abiotic conditions on Earth changed deeply, affecting evolving life.
Thus, we need to learn the basics of the history of the Earth.
Let us start from the outcome of this history, the current Earth, because "the present is the key to the past“. Uniformitarianism, based on the assumption that laws of nature did not change, has been first proposed for geology.
We will briefly review 4 subjects:
1) The Earth's orbit
2) Structure of the Earth
3) Plate tectonics
4) Types of rocks and the Rock Cycle
If the Sun and the Earth were dimensionless points, if the Earth's mass were zero, if there were no other plantes, and if there were no relativistic effects, the Earth's orbit would be described exactly by the Kepler's laws (remember them?).
However, this is not the case.
Quasiperiodic changes of the parameters of the Earth's orbit (Milankovich cycles) can be traced back, by solving Newton's equation for spheres, for ~20 Ma.
Changes in eccentricity, obliquity of the Earth's axis, and the positions of the equinoxes of the Earth's orbit. Lengths of time after which the dynamics of a parameter almost repeat themselves are shown in parentheses.
Comparison of the pattern of temperature changes inferred from sediments to "Solar (Astronomical, Orbital) Forcing", reverse-predicted from Milankovich cycling, shows that Solar Forcing is a major factor in long-term climate changes.
What sources of data do we have on the Earth’s inner structure? Geology studies present-day Earth uses essentially the same hypothetico-deductive method used for studying past life. Data on the inner structure of the Earth are from:
1) the Earth's magnetic field
3) emission of heat by the Earth
4) propagation of seismic waves.
There are several kinds of seismic waves which are “X-raying” the Earth after each earthquake. Too bad we cannot consider this fascinating subject in detail.
Outermost solid shell, 5-80 km thick, made of silicate rocks. Crust is separated from Mantle by Mohorovicic discontinuity (Moho) where velocity of seismic waves suddenly increases.
Extends from below the crust to 2900 km, and composed of silicate rocks that are also rich in iron and magnesium. The temperature increases from ~700°C at the upper boundary to >4,000°C at the lower boundary. Still, the mantle is almost exclusively solid, due to enormous lithostatic pressure. Still, the mantle is solid, due to enormous pressure. There are 3 distinct layers in the mantle:
1) Uppermost mantle - resistant to deformation, relatively cool, extends to ~100km. Together with the crust, forms the lithosphere.
2) Asthenosphere (weak sphere) - the least rigid layer of the mantle, which extends from 100 to 250km.
3) Lower mantle - again, a more rigid layer.
The innermost part of the Earth, consisting primarily from iron and nickel, and separated from mantle by a sharp boundary. Convection in the outer core gives rise to the Earth magnetic field. The core consists of:
1) Outer core - liquid
2) Inner core - solid
The lithosphere (crust plus uppermost mantle) exists as separate and distinct tectonic plates, which rides on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range from a few mm to about 15 cm per year. Dissipation of heat from the mantle is the original source of energy that drives plate tectonics.
Bridge across the Álfagjá rift valley in southwest Iceland, the boundary of the Eurasian and North American continental tectonic plates.
1. Transform - plates slide (grind) past each.
2. Divergent - two plates slide apart from each other, e. g., at mid-ocean.
3. Convergent - two plates slide towards each other, forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Examples are the Andes mountain range in South America and the Japanese island arc.
1) Igneous rocks - solidified from hot silicate melt, magma, or lava. Small crystals indicate fast cooling. Igneous rocks are suitable for radiometric determination of age.
2) Sedimentary rocks - layers of deposited particles on Earth surface (water, wind, ice). Fossils are preserved mostly within sedimentary rocks.
3) Metamorphic rocks - rock that changed, after primary formation.
Rock Cycle: Igneous rocks form directly by crystallization of hot melts made up of silicates combined with Fe, Mg, Ca, Al, Na, K, Ti, and H2O. They make up nearly all the mantle and crust. Rocks at the surface decompose/disintegrate by reaction with the atmosphere/hydrosphere to produce solid debris and soluble chemicals (sediments), that upon burial are converted to Sedimentary rocks. Previously formed rocks that are heated and pressurized when buried to shallow to moderate depths (5 to 70 km) of the crust recrystallize as solids to form Metamorphic rocks (some may melt). These processes comprise the Rock Cycle.
Of course, there is no real periodicity here, and an igneous or a sedimentary rock may persist for billions of years.
How humble biologists defeated a proud physicist
In 1862, Lord Kelvin used Fourier's theory of heat conductivity to calculate the age of the Earth. He knew the Earth's temperature increased one degree Fahrenheit for each 50 feet you went into the ground. He guessed that the Earth began as molten rock at 7000°F. By solving Fourier's equation, Kelvin found that it must have taken a hundred million years for the Earth's temperature to level out to one degree every 50 feet. Kelvin concluded that the Earth was far too young to satisfy geologists and Darwinists.
What was wrong with this analysis?
Why does the Earth remain hot?
Now we are ready to review the history of the Earth
There is a number of methods for establishing the ages of rocks:
a) Radiometric – by measuring concentrations of a parent (radioactive) and a daughter (stable) isotope, in pairs such as:
238U/206Pb, 235U/207Pb, 40K/40Ar, 87Rb/87Sr, 147Sm/143Nd, 187Re/187Os, 10Be/10B, 14C/14N.
Big Bang did not produce atoms heavier than 7Li, and nucleosynthesis within ordinary stars does not produce atoms heavier than 56Fe. All heavier atoms were synthesized in supernovae explosions. Our Solar system belongs to a second-generation star. How can Pb be a daughter of U?
b) Measuring thermoluminescence.
c) Amino acid racemization.
d) Relating patterns in sediments to Milankovitch cycles of the Earth orbit.
e) Relating data on paleomagnetism to reversals of the Earth magnetic poles.
Methods a), b), c), and d) are all based on “playing time back”, by studying an irreversible deterministic process.
Knowing concentrations of parent (P) and daughter (D) isotopes within a crystal, we can play back the dynamics of radioactive decay and, thus, calculate the age of the crystal t. Indeed, we know that P declines at rate proportional to P, with the coefficient of proportionality l (decay constant).
1-dimensional phase space of the number P of radioactive atoms in a sample.
Blue line: rate of P as a function of P:
dP/dt = -lP
Red lines: P as functions of time t, depending on the initial number of atoms P0:
P(t) = P0e-lt
where half-life t = ln(2)/l.
Radioactive decay is a (macroscopically) deterministic process - knowing the current number of atoms, we can predict this number for any moment in the future, and reconstruct it for any moment in the past.
In particular, if the number of radioactive parent atoms is equal to the number of stable daughter atoms, the age of the crystal is t years, where t is half-life.
l can be measured now - due to enormous sensitivity of measurements of radioactive decay - and is almost independent of anything. 270 Kbars pressure increases l for electron capture 7Be > 7Li decay by only 0.6%.
indeed describes exponential decline of P:
Let us solve this equation, using the following general theorem: Generally, a solution x(t) of the autonomous differential equation dx/dt = f(x) which satisfies the condition that at some moment of time t0 the value of the dependent variable x was x0 can be found by integrating this equation:
This excercise prepares you for solving a slightly more complex equation, which describes a selection-driven allele replacement, the key mechanism of evolution:
Radiometric dating is the most important method of determining the age of rocks. Potassium-argon dating based on the 40K/40Ar isotope pair is particularly important.
Potassium (K) exists in 3 isotopes - 39K (93.2581%), 40K (0.0117%), 41K (6.7302%). The radioactive isotope 40K decays to 40Ar and 40Ca with a half-life of 1.26x109 years. After the rock solidifies, radiogenic 40Ar will begin to accumulate in the crystal lattices. K and Ar can be measured separately, but this is not the best option.
Argon-argon method is a variety of potassium-argon dating. The sample is first irradiated in a nuclear reactor and a known fraction of 39K is converted to 39Ar. The relative abundances of the radiogenic daughter 40Ar and of 39Ar (a proxy for the parent 40K) are measured in the same sample by a mass spectrometer, with a very high precision. In particular, argon-argon method was successfully used to determine the (known) age of volcanic ash that buried Pompeii in 79 CE, although only a tiny amount of 40Ar accumulated since then.
Radiocarbon method, based on the presence of a known fraction of radioactive 14C in atmospheric CO2 and, thus, in all "live" C, can be used to establish the age of relatively recent organic matter, because the half-life of 14C is only 5730 years. Radiocarbon dates can be affected by contamination with the younger organic matter.
Another variety of radiogenic dating: counting the damage tracks left by fragments of 238U nuclei that underwent spontaneous fission. As always, the decay rate is known, and the concentration of parent nuclide has to be measured.
Natural crystals contain "traps" (defects) which can hold electons excited by radiation. When a crystal is heated, these trapped electrons are released, emitting a weak light, proportional to the total dose of radiation accumulated by the crystal since the moment when it was last heated or exposed to light. This dose is a proxy of time.
Amino acid racemization is the process of interconversion of amino acids from one chiral form (the L - laevo amino acids which are the building blocks of proteins) to a mixture of L- and D- (dextro) forms following protein degradation.
The observed proportion of D-amino acids (a function of time and temperature) can be used to estimate the age of a sample.
Comparison of the pattern of Solar Forcing reverse-predicted from Milankovich cycling to changes of the temperature recorded in sediments can be used for dating sediments that are younger than ~20 Ma.
e) Relating paleomagnetism to reversals of the Earth magnetic poles
Occasionally, the Earth's magnetic poles switch their locations, and the global record of these switches is well-established.
Such switches occur very rapidly, on the geological scale. Still, occassionally the movements of the poles during a transition can be traced.
Sedimentary rocks are especially important for paleontology. Continuous deposition of sediments may occur for many millions of years. Interpretation of sequences of sedimentary rocks are a subject of stratigraphy and are based on these principles (proposed by the Danish geologist Nicholas Steno, 1638-1686):
1) Superposition: in a sequence of layered rocks, layers are arranged in a time sequence, with the oldest on the bottom and the youngest on the top, unless later processes disturb this arrangement.
2) Original horizontality: layers of sediment are originally deposited horizontally.
3) Lateral continuity: layers of sediment initially extend laterally in all directions. Thus, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
4) Biotic succession: different strata contain particular types of fossilized flora and fauna, and these fossil forms succeed each other in a specific order.
Of course, these principles are based on uniformitarianism - but not on gradualism. Indeed, interpretation of sedimentary rocks can be complicated by drastic patterns of the following kinds:
1) hiatus (no deposition for a while)
2) angular unconformity (different angles in two superpositional rocks)
An angular unconformity between Tertiary sedimentary rocks (tilted beds) and loosely consolidated Quaternary conglomerates and sandstones (horizontal beds) near Pacific Beach on the Olympic Peninsula, Washington.
Flat-lying sedimentary rocks from Kaibab Limestone (Permian) at top to Bright Angel Shale (Cambrian) at base of section.
Ripple Marks on sandstone of the Triassic Chinle Formation - sometimes fine details are preserved in sedimentary rocks.
Stratigraphy can reconstruct successions of events that produced even very complex successions of layers of sedimentary rocks.
Composite stratigraphy of the Hadar Formation, with the positions of hominid fossils indicated by asterisks.
The face of the Earth is constantly changing. Even major features, including locations of oceans and continents and their relief, are not invariant. Several methods are used to study paleogeography:
1) Tectonic reconstructions, based on knowledge of movements of tectonic plates,
2) Data on paleomagnetism, which reveal orientation of forming rocks relative to the contemporary positions of magnetic poles,
3) Data on local paleoclimates, which may provide information on the geographical latitude of a particular location in the past,
4) Biological correlations (provincialism), which reveal ancient continuity of ranges of species.
This last method illustrates, together with the principle of biotic succession, the utility of fossils for all Earth Sciences. The idea is simple: lands populated by the same (or tightly related) organisms at some moment in the past must have been in close proximity to each other at that moment.
Glossopteris was a seed fern that was widely distrubuted over the southern land masses during the Permian and Triassic periods (roughly 300 to 200 mya).
Distribution of the Permian Glossopteris flora reveals a continuous Southern continent, Gondwana.
Climate underwent profound changes, both globally and locally. Data on paleoclimates are provided by ratios of stable isotopes of the same element.
Some important ratios are 18O/16O, 11B/10B, 13C/12C, 87Sr/86Sr, and 98Mo/95Mo.
For example, there are three stable isotopes of oxygen, 16O, 17O, and 18O, and the ratio of molar concentrations of 16O and 18O is conventionally characterized by
d18O = (18O:16Osample - 18O:16Ostandard)/18O:16Ostandard x 1000
where 18O:16Ostandard is 2005.20.
d18O in sediments depends on a number of factors, mostly because a lighter H216O evaporates at a higher rate. Thus, d18O in a sedimentary rock or ancient ice is affected by the overall volume of ice on the Earth at the corresponding time (the ice is enriched by 18O, so that when a lot of ice is present, the remaining water is enriched with 16O) and by the temperature at a site of the deposition.
A drastic temporary reduction in d13C at the boundary between Neoproterozoic and Paleozoic eons (and Ediacaran and Cambrian periods) indicates rapid changes of the global environment (perhaps, oxygenation of deep Ocean), which probably triggered the replacement of Ediacaran biota with Cambrian biota.
There were many drastic global events in the history of the Earth - some of which took thousands of years and some others being trully sudden. Possible signs of a past disaster due to an extraterrestrial impact:
b) iridium anomaly
c) shocked quartz and microtektites of impact-melt glass
d) tsunami deposits
Vredefort in South Africa, the oldest and largest of known impact craters is 2,023 Ma old and 250-300km in diameter.
Chicxulub is an impact crater buried partially underneath the Yucatán Peninsula and partially under the sea. Reconstruction of the crater (left) and its structure revealed by slight changes in the intensity of gravitation on the surface, which reflect different degrees of compression of underlying rocks (right).
Ir is found in concentrations up to 100 times normal in many thin geological layes world-wide. As Ir is much more abundant in asteroids and comet bodies, an Iridium Anomaly is an evidence of a
There are several well-studied iridium anomalies, uncluding the one at the Cretaceous-Tertiary (K-T) boundary 65 mya, due to the Chicxulub impact.
Ir anomaly occurred at the Triassic-Jurassic boundary 200Mya.
Shocked quartz is a form of quartz that has a peculiar microscopic structure. Under intense pressure (but limited temperature), the crystalline structure of quartz will be deformed along planes inside the crystal. These planes show up as lines under a microscope, which are called shock lamellae.
Microtektites are submillimeter-size dark-colored, rounded silicate glass particles, often aerodynamically shaped. Microtektites form by impact melting of terrestrial sediments.
Debris flows, exotic boulders, and turbulence features associated with 3,470 Ma old asteroid impact triggered-tsunamis, Pilbara Craton, Western Australia.
a) Usually fossils are found in sedimentary rocks (so a fossil must be found between layers of igneous rocks, for a reliable direct determination of its age)
b) Fossils can be formed only under rather restrictive conditions (fast burial, no oxygen, etc.). Fossilization is a complex chemical process.
c) Nevertheless, fossils are very abundant. There are ~250,000 described fossil "species".
d) There are body (micro and macro) and trace fossils.
e) As fossils show that life evolved, they are direct evidence for past evolution.
f) Fossils is the only source of information on those ancient forms of life that left no descendants (below).
A "routine" path to fossilization (top left). "Lyuba", a baby mammoth preserved in permafrost for ~40,000 years (top right). Spider fossil in amber (bottom left). An assembly of crustaceans from the order Malacostraca found in Montceau-les-Mines Lagerstätte in France (bottom right).
After fossilization, profound changes can occur, similar to diagenesis of sediments.
Cross-section of a petrified tree trunk, in which all the organic material have been replaced with silicates (left).
A permineralized trilobite , Asaphus kowalewskii (right).
However, sometimes organic matter, including macromolecules, can be preserved in fossils.
Micrograph of vessels and matrix in demineralized bones (top left) of Brachylophosaurus canadensis, a 80-million-year-old dinosaur (top right). A phylogeny, based on collagen sequences of three extinct (two dinosaurs and mastodon Mammut; boldfaced) and a number of extant species (bottom).
Proterozoic bacterium (top left). 60 Ma old radiolarians, unicellular eukaryotes (top right). Microscope picture of a 460 Ma old fungal spore with attached hypha (middle left). A print fossil of unclear affinities Dickinsonia, found in the Ediacaran (middle right). Ichthyosaur (bottom left). Fossil ferns from the Permian (bottom right). 3.5 Ma old Australopithecus afarensis, very likely our direct ancestor (very bottom).
Vertebrate traces in the Permian Coconino Sandstone, Arizona (left).
A fossilized dwelling burrow, probably of a marine crustacean from the Cretaceous Bad Heart Formation in Alberta (right).
a, Parapandorina fossils from the Doushantuo, China have features akin to those of animal embryos. This image of a two-cell Parapandorina 'embryo' was derived using X-ray microtomography. The cells' interiors are dominated by what seem to be membrane-bound vacuoles or yolk particles.
b, However, the fossils could be bacteria similar to Thiomargarita (right). The bacterium shown has divided into four cells, in which the vacuoles are clearly visible.
For some organisms, in particular, marine plankton with hard shells, fossil record can be very detailed and reveal gradual anagenesis ...
In other cases, however, fossil record is very incomplete, and establishing exact ancestor-descendant relationships may be difficult. An example of such incomplete record is provided by known fossils that span the gap between sarcopterygians and amphibians.
A correspondence between the timing of the first appearance of a trait state in the fossil record and its position within the hierarchy of traits of extant organisms.
Trait states that appeared later are nested within trait states that appeared earlier.
Useful concepts: crown and stem groups. All organisms, extant and extinct, which originated from the most recent common ancestor of extant birds belong to the crown group of birds. They include both direct ancestors of some modern bird species and extinct forms that left no descendants. Extinct organisms that diverged before the last common ancestor of living birds but still are more tightly related to birds than to any other living organisms belong to the stem group of birds. Together, the crown and the stem groups of a clade of living species comprise its total group.
In the past, global climate underwent many deep changes, caused by orbital variations, continental drift, changes in the Sun luminosity, changes in volcanism (caused by plate tectonics), changes in the atmosphere composition, and extraterrestrial impacts. As the result, periods of warm temperatures alternated with Ice Ages, where a substantial fraction of Earth was covered by ice.
Average global temperature, relative to the current one, estimated through d18O, during the Phanerozoic eon.
Well-established Ice Ages include:
1) Huronian, 2,400-2,100 mya;
2) Cryogenian, 850-630 mya. This Ice Age included two major glaciations, Sturtian (725-710 mya) and the Marinoan (660-635 mya);
3) Andean-Saharan, 460-430 mya;
4) Karoo, 330-260 mya;
5) Pliocene-Quaternary, which started 2.58 mya and continues until the present, with glaciations alternating with periods of higher temperatures.
1) ~1100 mya, most of Earth's landmass formed one supercontinent, Rodinia, which broke up ~750 mya, probably into three continents.
2) ~600 mya, a single supercontinent Pannotia was formed , to break up 540 mya, probably into four continents.
3) ~300 mya, a single supercontinent Pangea appeared again.
4) The break-up of Pangea began ~200 mya, first into two supercontinents (Gondwana to the south and Laurasia to the north), thereafter into the continents that exist today.
Durin the history of the Earth, there were times when ocean waters contained high concentrations of dissolved Fe2+ (ferruginous conditions), H2S (sulfidic, or euxinic conditions), and O2 (oxic conditions). To complicate the matters further, the ocean was often stratified, with different layers of water having very different properties.
Initially, the oxygen released by cyanobacteria reacted with oceanic Fe2+, to form insoluble iron oxides Fe2O3 and Fe3O4. These oxides were deposited on the ocean floor as banded iron formations (BIFs). Accumulation of BIFs was very common between ~2,500Mya and ~1,800Mya, after which it almost stopped for ~1000 Ma.
The BIF, deposited at the sea bottom 2,700 mya, was folded by the movements of plates (left). Some prominent BIFs, used as iron ore (right).
It seems plausible that the ultralow-velocity zones observed floating on the Earth's core may be ancient BIFs that sunk there through the mantle.
Probable history of chemical conditions in oceans (OMZ, oxygen minimum zone, is a layer of water below surface waters). Surface waters became oxygenated ~2,400 mya. Dashed lines represent postulated or uncertain histories. Vertical lines represent Sturtian Marinoan, and Gaskiers glaciations (left to right). Sulfidic deep waters for ~1000 Ma between the 1st and the 2nd periods of accumulation of BIFs was likely due to prevalence of anoxygenic photosynthesis below surface waters. Oxygenation of middle waters after Gaskiers glaciation and of deep waters after the Ediacaran period, probably due to changed patterns of circulation, apparently triggered diversification of animals and the origin of Cambrian biota.
Changes in the concentration of oxygen in the atmosphere deeply affected evolving life and were caused to a large extent by the activity of living beings. Presumably, the atmosphere of the primordial Earth mostly consisted of H2 and He. There gases were rapidly lost into space, because the Earth's gravity is not strong enough to hold them, and replaced by volcanic gases.
O2 build-up in Earth's atmosphere (upper and lower curves represent the range of estimates): 1) 3,850–2,400Mya: no O2, 2) 2,400–1,800Mya: O2 produced but reached only a low concentration, being mostly absorbed in oceans and seabed rock, 3) 1,800–0,800Mya: O2 produced but mostly absorbed by land surfaces and formation of ozone layer, 4) 800–540Mya: O2 sinks filled and the gas accumulates, 5) 540 mya – present: near-current concentrations of O2.
Concentration of CO2 also changed substantially in the past, affecting global climate due to greenhouse effect. The end of Cryogenian Ice Age may be caused a a rapid increate of the CO2 concentration, perhaps due to global release of methane from low-latitude permafrost. More reliable inferences on CO2 concentration are currently available for the Cenozoic era.
Atmospheric CO2 concentrations were above 2,000 p.p.m. from 60 to 52 mya and declined after that, perhaps due to reduced CO2 outgassing from ocean ridges and volcanoes and increased carbon burial. Since 24 mya, atmospheric CO2 concentrations stayed below 500 p.p.m. These conclusions are based on estimates of the pH of ocean water from the d11B in fossil foraminifer shells.
for legible figures.
Geological time scale organizes all the data on history of the Earth and life on it.
ARCHEAN eon: 3,850 - 2,500
Before 3,500: Origin of life
3,466-3,430: Earliest fossils of prokaryotes, including stromatolites
2,900: Likely origin of cyanobacteria and oxygenic photosynthesis
2,600: The first likely traces of terrestrial prokaryotes
PROTEROZOIC eon: 2,500 - 542
PALEOPROTEROZOIC era 2,500 - 1,600
2,500-1,840: Massive accumulation of Banded Iron Formations (BIFs)
2,400-2,320 Substantial accumulation of previously absent O2 in the atmosphere
2,400-2,100: Huronian Ice Age
2,023: Vredefort, the oldest and largest (diameter ~300km) impact crater known
1,800: The first plausible fossils of unicellular eukaryotes
1,800-800: Probably, high concentrations of H2S in deep oceanic waters
MESOPROTEROZOIC era 1,600 - 1,000
1,300: Abundant fossils of eukaryotes, including some multicellular forms
1,100-750: Supercontinent Rodinia
NEOPROTEROZOIC era 1,000 - 542
Tonian period: 1000-850
850: Diversification of multicellular eukaryotes
Cryogenian period: 850-635
800-600: Second period of accumulation of BIFs
800-640: Initial diversification of animals
725-710: Sturtian glaciation
660-635: Marinoan glaciation
Ediacaran period: 635-542
600-540: Supercontinent Pannotia
580: Gaskiers glaciation
575-542: Ediacaran biota, the first record of diverse macroscopic animals
PALEOZOIC era: 542-251
Cambrian period: 542-488
542-520: First fossils of almost all phyla of animals
Ordovician period: 488-444
460-430: Andean-Saharan Ice Age
450: First known liverwort fossils
Silurian period: 444-416
425: First known vascular plant fossils
420: Likely origin of jawed fishes
Devonian period: 416-359
400: First known insect fossils
380-365: Origin of tetrapods
Carboniferous period: 359-299
360-260: Karoo Ice Age
314: First definite amniote fossils
300-200: Supercontinent Pangea
Permian period: 299-252
265: Origin of therapsids, mammal-like reptiles
251: End-Permian mass extinction
MESOZOIC era: 252-65
Triassic period: 252-202
230: Origin of dinosaurs
Jurassic period: 202-146
200: Break-up of Pangea into Gondwana and Laurasia
170: Diversification of mammaliforms and early mammals
150: Oldest known bird fossils
Cretaceous period: 146-65
125: First known fossils of flowering plants
125: First known fossils of placental mammals
65: KT mass extinction
CENOZOIC era: 65-present
~6: Divergence of Pan and Homo lineages
2.58-present: Pliocene-Quaternary Ice Age
Draw a chart which displays ~15 important milestones (definite and conjectural) in the history of abiotic conditions on Earth, with times of their occurrence and brief descriptions - and memorize these milestones!