Earth has a strong magnetic field (Figure 3l), and a long history of plate tectonics dating back to Precambrian times. It is at a distance of 1 AU from the Sun. Earth and Venus are about the same size (Figure 3m), but their surfaces are totally different because Venus had an entirely different geological evolution as discussed earlier. Earth’s oceans comprise about 70% of the surface (Figure 3n).
The origin of Earth’s water has been attributed the Late Heavy Bombardment (LHB) by comets and water-rich asteroids about 3.9-4.1 billion years ago. However, there is evidence from pre-Cambrian zircons that Earth had oceans as early as 4.3 billion years ago, or at least 200 million years before the LHB [14
]. This would indicate the water was present well before the LHB and was intrinsic to Earth. Furthermore, a recent study [15
] of deuterium-to-hydrogen enrichments finds that the solar nebula had abundant water ice that was available for comet and planetary water formation. Recent studies of Comet 67P/Churyumov–Gerasimenko by the ESA Rosetta mission indicate that its water is much richer in deuterium than Earth’s water. This seems to indicate that comets are not the source of Earth’s water. In this study we consider Earth’s water to be derived primarily from the solar nebula, but additions probably occurred from comets and water-rich asteroids.
Life probably evolved in the early oceans from compounds of the six essential elements required of all life; carbon, hydrogen, oxygen, phosphorus, sulfur and nitrogen. We do not know how life originated from these non-organic compounds, but the energy was probably provided by photosynthesis. The earliest most primitive life on Earth was prokaryotes (unicellular organisms whose cells lack a membrane-bound nucleus). They were present about 3.6 billion years ago (Ga). Life may have occurred earlier, but that record has not yet been found. The first complex life (cyanobacteria using photosynthesis) began about 3.5 Ga. Complex cells (eukaryotes
) began about 2 Ga and multicellular life started about 1 Ga. Complex animals (animals with a back and front) began about 550 Ma. From this time evolution happened rapidly during the last 12% of Earth history. Land plants began about 475 million years ago, and amphibians about 360 million years ago with the first migration and adaption to land. Reptiles began about 300 million years ago, and mammals about 200 million years ago.
There have been 5 mass extinctions where greater than 50% of species became extinct. At least two were caused by climate changes. The largest mass extinction was the Permian/Triassic extinction that caused ~95% of the species to perish about 252 million years ago. It was caused be the massive eruption of the Siberian flood basalts that covered an area about the size of the United States. The eruptions resulted in the release of massive amounts of greenhouse gases that drastically warmed the planet and drastically acidified the oceans over a period of about 10,000 years.
The mass extinction at the Cretaceous/Tertiary boundary (K/T extinction) 66 million years ago was also caused by a climate change that killed 76% of all species including the non- avian dinosaurs. This extinction was probably caused by a large impact of a ~10 km asteroid that formed the 180 km diameter Chicxulub crater on the Yucatan peninsula of Mexico. The impact probably triggered the enormous Wai Subgroup of Deccan flood basalt lava flows, which may account for >70% of the Deccan Traps main-stage eruptions [16
]. This eruption of the Deccan flood basalts is the same age at the Chicxulub impact [17
], and released massive amount of CO2
. This mass extinction of 76% of all species was a result of a rapid extreme climate change caused by both events. However, it was probably due mostly to the impact because the Earth was already in a Hot House, and the size of the Deccan eruptions (500,000 km2
) was considerably less than the Siberian eruptions (2-7 million km2
) that caused the Permian/Triassic mass extinction. Furthermore, the impact first caused warming, and then a major cooling for about 10 years with temperatures equivalent to an Ice Age.
During the impact there was first an extreme heating of the atmosphere by the entry of impact ejecta. This was followed by freezing conditions for about 10 years from huge amounts of atmospheric aerosols ejected into the stratosphere that reflected sunlight back to space. At that time the planet was in what is called a “Hot House” with global average temperatures about 9°C warmer than today. The climate change from very hot to ice age temperatures resulted in the extinction of the non-avian dinosaurs, but some mammals survived because they were burrowing animals that largely avoided the fatal effects of extreme climate change. Furthermore, their greatest predator, the dinosaurs, had been eliminated by the climate change. After the demise of the dinosaurs, the mammals flourished in the Cenozoic
era known as the “Age of Mammals”. If that extinction had not happened dinosaurs would still be here devouring mammals and keeping them in a relatively primitive state, and almost certainly we would not be here today. We owe our existence to the K/T extinction that occurred 66 million years ago.
A new study indicates that Earth has entered its 6th mass extinction event [18
]. These results are consistent with earlier studies by Duke University and E. Kolbert’s book titled “The Sixth Extinction”. The new study found that vertebrates are disappearing at a rate 114 times faster than when Earth was not going through a mass extinction. It could be 1000 times faster. From 1900 to present, more than 400 vertebrate species have become extinct. Normally this type of loss would take about 10,000 years. About 41% of amphibians and 25% of mammals face extinction. According to the lead author of this new report, “If it is allowed to continue, life would take many millions of years to recover and our species itself would likely disappear early on”. The current extinctions are the result of climate change, deforestation, pollution, and over population.
Below are two tables where the geologic timescale and human evolution are shown relative to one year. This puts in perspective our species evolution compared to the geologic timescale.
We know less about the Precambrian Eon, which comprises 88% of geologic time, than any other time in geologic history. On the geologic time scale relative to one year it lasted 10 months and 17 days from Jan. 1 to Nov. 18. The Cenozoic Era that started after the 5th mass extinction and includes human evolution comprises only 1.4% of geologic time. Relative to one year it started on Dec. 26th and has duration of only 5 days. Human history (Homogenus) began ~2.8 million years ago, or the early evening of the last day of the year on a time scale relative to 1 year (Table 3).
It is important to understand that our technology occurred very late on the geologic time scale and that other present technological civilizations in our galaxy probably developed much earlier than ours. Therefore, they should be much more advanced than our technology. On a geologic time scale relative to one year, our technology began 1.7 seconds ago and our manned exploration of space began only about 0.2 seconds ago (Table 3).
Human evolution began about 7 million years ago, and our species began about 125,000 years ago. Civilization began about 12,000 years ago and the Industrial Revolution (Technology) began only 250 years ago. We began radio transmission that would have been emitted to the rest of the Universe only 92 years ago. That signal of our existence has now traveled only 92 LY into our galaxy (Figure 2f). Since our galaxy is about 110,000 LY in diameter, our signal has only traveled 0.0008% of the galactic diameter.
On Earth it took 4.56 billion years for humans to evolve and develop technology. During this time 99.9% of all species that ever inhabited the Earth became extinct. We are here because a mass extinction killed the dinosaurs and allowed us to evolve from the mammals that flourished after the impact. Will we be the exception and escape extinction, or will we also become extinct like the vast majority of other species? Currently Earth apparently has entered the 6th mass extinction due to climate change, deforestation, pollution and over population. Unless we do something to stop these conditions soon, it is likely we will become extinct in the not too distant future.