The first “intimate encounter” likely occurred around 1.2 billion years ago. While our blue planet, home to over 7 billion people, once resembled a snowball like some of Jupiter’s moons, surprisingly, life still managed to sprout from such environments.
Important Milestones in Earth’s History
1. The Birth of Earth | 4.5 Billion Years Ago
The first major event in the life of a planet is, of course, when it “cries out” for the first time. Formed from a cloud of gas and dust scattered around a still young Sun, Earth grew as masses of rock collided and acquired enough mass to attract surrounding dust and gas through gravitational forces. Earth’s only satellite, the Moon, came into existence soon after. It is believed that a massive asteroid collided with Earth, sending debris flying into space, which gradually coalesced to form the “Lady Moon” orbiting “Mr. Earth.”
Analysis of lunar rocks indicates that the Moon and Earth share a common origin due to their similar chemical compositions.
2. The First Signs of Life | 4 – 3.5 Billion Years Ago
No one knows exactly when the first cell appeared. However, the oldest known fossil dates back 3.5 billion years. Thus, the first signs of life likely emerged even earlier, although certainly not while this planet was still a molten ball bombarded by countless meteors. It is believed that the first life forms began in warm alkaline hydrothermal vents on the ocean floor, or possibly at the surface of the sea or on land.
Asexual reproduction (cell division) was the first form of reproduction
We still lack sufficient data to know what the first life forms looked like.
3. Harnessing Solar Energy | 3.4 Billion Years Ago
Although humanity has only begun to utilize solar energy in recent decades, nature has known this concept for a long time. Naturally, all life requires energy, and the closest source of energy to Earth is the Sun. The first microorganisms evolved under sunlight, using energy from photons to synthesize sugars from simple molecules, a process we call photosynthesis.
The first bacteria synthesized sugar from solar energy but did not produce oxygen
However, unlike today’s plants, these early microorganisms did not “release” oxygen, making Earth’s atmosphere at that time still “toxic” to humans and other life forms.
4. The “Migration” of Continents | 3 Billion Years Ago
Though seemingly solid and “unchanging,” the Earth’s crust is actually made up of “floating icebergs” on molten magma. Occasionally, these “icebergs” collide, causing one to “sink” beneath another. This process is known as plate tectonics or the drift of continents. The first continent on Earth, known as Ur, was formed through this process.
The Earth’s surface is essentially a “patchwork” of tectonic plates
5. The First “Pollution” of the Atmosphere or Major Oxidation Event | 2.4 Billion Years Ago
The first photosynthetic reactions occurred over 1 billion years ago, but they did not produce oxygen. During the first half of Earth’s life, there was very little oxygen in the atmosphere. Nevertheless, some bacteria somehow learned to synthesize sugars from CO2 and water, of course with the aid of sunlight. The oxygen was a “waste product” from these bacterial “factories“.
Oxygen flooding the atmosphere is the first pollution in Earth’s history
The first “pollution” event in Earth’s history is considered to be the reason why the entire planet later experienced a “glaciation”, as the bacteria “sucked out” greenhouse gases (including CO2) from the atmosphere, causing everything to cool down.
6. Complex Cells and Symbiosis | 2 – 1 Billion Years Ago
Although life had existed on Earth for a considerable time, it remained very simple. The first cells had structures similar to modern bacteria. However, evolution propelled life to a new level. Organisms known as eukaryotes (organisms with a nucleus) developed more complex structures with specialized organelles inside, and a membrane-bound nucleus separate from the rest of the cell.
Mitochondria are actually bacteria capable of synthesizing ATP that were “swallowed” by other microorganisms
Eukaryotes also found for themselves new “powerhouses”. These structures, shaped like beans, are known as mitochondria and are capable of converting energy from organic matter into ATP.
It is believed that initially, mitochondria did not naturally arise within cells but were separate bacteria living in the outside world. However, through symbiosis or cellular absorption, these bacteria “shared a roof” with eukaryotes and provided them with energy. The cells of all animals and plants we see today are eukaryotic cells.
7. The First Sexual Reproduction | 1.2 Billion Years Ago
The earliest organisms reproduced asexually, with cells growing and then automatically dividing without distinguishing between “male and female.” Fossils dating from 800 million to 1.8 billion years ago are quite unremarkable. Scientists refer to this period as the “Boring Billion.” However, it seems something went “off course” from the traditional method of reproduction.
The emergence of sexuality marks a turning point for future species hybridization
Fossils dating back 1.2 billion years of red algae show that specialized sex cells, such as spores, began to emerge. Since these fossils appeared after the organisms’ inception, sexual reproduction must have originated before this time.
8. Multicellular Organisms | 1 Billion Years Ago
Eukaryotes represented a significant advancement in life, but things did not stop there. To achieve larger sizes, organisms needed more than one single cell. Fossils dating back 2.1 billion years indicate that many bacteria had learned to live communally, yet they could still exist as unicellular organisms when necessary.
Bacterial populations living in “herds” may have been the beginning of multicellular organisms
The first multicellular organisms – with cells that could not separate – are thought to have emerged around 1 billion years ago. Within this, different groups of organisms evolved towards multicellularity independently. Plants are believed to have adopted multicellularity before animals.
9. Snowball Earth | 850 – 635 Million Years Ago
For the second time, Earth underwent a process of becoming a frozen wasteland after the first instance of oxygen “pollution” from bacteria billions of years earlier. The reasons for this freezing event remain unclear, but it lasted for 200 million years, with ice covering the planet from the poles all the way to the equator.
Earth once experienced a “cold and icy” period lasting millions of years
This second ice age is believed to have “encouraged” the emergence of the first complex life forms. Organisms with tubular or frond-like structures, similar to ferns, appeared soon after, leading to a new era known as the Ediacaran Period.
10. The Cambrian Explosion | 535 Million Years Ago
After animals evolved and adapted to new conditions, life forms witnessed two significant explosions in species diversity.
During the Cambrian Explosion, new groups of animal species emerged approximately every 10 million years. However, this “explosion” was largely due to the fact that most animals of this era had hard shells, making fossilization much easier. It is likely that there were previous “explosions” of life, but they went unrecorded by science because these species lacked hard structures.
Hard shells facilitate easier fossilization
Forty-five million years later, life forms recorded another explosion in species diversity across various animal groups, known as the Ordovician Biodiversification Event.
11. Plants Invade Land | 465 Million Years Ago
Water remained the cradle of life on Earth for billions of years. It was not until about 500 million years ago that some animal species began to venture onto land. However, they did not live there but merely “strolled” or sought places to lay eggs—essentially to avoid predators lurking in the water.
Green algae were the first “residents” on land
Plants, on the other hand, became the first “permanent residents” of terrestrial environments. The first plants to claim territory were relatives of green algae, which quickly evolved and branched out into many different species.
12. The First Mass Extinction Event | 460 – 430 Million Years Ago
The Ordovician Period was a time of flourishing biodiversity. However, as this period came to a close, the world faced a mass extinction event that rivaled the number of species that had emerged. By the end of the Ordovician, Earth experienced rapid cooling, with ice from the poles covering most of the planet’s surface. This led to an ice age known as the Andean-Saharan glaciation, named for its remnants found on the peaks of the Andes as well as in the Sahara desert.
Major extinction events on Earth are often linked to ice ages
This frigid period marked the first and second largest mass extinction event on record. It is estimated that up to 85% of marine species were wiped out, and since most life forms were still aquatic, the severity of the Andean-Saharan extinction is not surprising. Consequently, fish species thrived as competitors disappeared.
