Let’s consider how long planet Earth has existed, when life first appeared, when we arrived, and what life has been through to get this far.
To do that, we turn to radiometric dating and the fossil record.
From radiometric dating – the rate of radioactive decay is constant over time – we know Earth has existed for about 4.56 billion years.
Our galaxy – the Milky Way – has existed for 13.2 billion years in a universe 13.8 billion years old.
According to the fossil record, which is a trace of an organism of a past geologic age, life in the form of primitive single-cell microorganisms began on Earth about 3.8 billion years ago.
It took more than 3 billion years before the first multi-cellular plants and animals appeared.
That was about 670 million years ago.
About 525 million years ago in the Paleozoic Era, the Age of Vertebrates and Invertebrates began.
This age, which lasted nearly 300 million years, yielded insects and the beginning of fish and reptiles.
About 245 million years ago in the Mesozoic Era, the Age of Reptiles began.
This was the time of the dinosaurs.
It lasted for about 180 million years.
Compare that to how long we have been here:
The family of humans – hominids – have been here about six million years.
We – modern humans – have been here about 300,000 years.
The Age of Reptiles ended about 65 million years ago when a flaming asteroid described to be the size of a city – 7 to 50 miles wide – struck Earth in an area now called Chicxulub, Mexico on Mexico’s Yucatan Peninsula on the Gulf of Mexico not far from the United States.
It struck with a force equivalent to ten billion times the energy of the atom bomb dropped on Hiroshima.
The explosion set Earth on fire, caused mile-high tsunamis, covered the planet with debris, blocked out the sun for years, destroyed all the plant material, killed off the dinosaurs, giant marine reptiles, and 75 to 80% of life, and ended the Age of Reptiles.
That began the Cenozoic Era sometimes called Age of Mammals although some mammals first appeared more than 200 million years ago.
Mammals are a hairy species that feed their young from mammary glands.
Mammals range in size from shrews that weigh 1/14 ounce (2 g) to whales that weigh 140 tons (127,000 kg).
The Age of Mammals is the age in which we live.
We are one of about 5,500 species of mammals.
In mammals, we see the beginning of the shift from the reptilian brain to the more advanced brain.
Reptiles hatch from eggs.
Although some reptiles tend their nest, most reptiles do not take care of their eggs.
Young reptiles are able to feed and care for themselves.
By contrast, mammals nurture their young.
Great emphasis is placed on feeding, protecting, and teaching skills to the young in order for them to survive on their own.
As a consequence of this behavior, the brain began to evolve from the reptilian dinosaur instinctive brain to the limbic system of the brain (emotions and feelings), to the neocortex (the rational and thinking brain).
Mammals possess the most highly developed brain of all animals.
A large brain size in relation to their body and the brain structure known as the cerebral cortex – the brain’s outer layer – provide mammals with highly developed intelligence and senses that increase their ability to learn and perform detailed tasks.
What about us?
How long have modern humans been here?
It is thought that about five to six million years ago the succession of species that gave rise to us separated from the succession that led to the apes.
From a common ape ancestor, the apes moved off in one direction and we, the hominids or family of humans, moved off in another.
The first genus of the hominids, five to six million years ago, was Sahelanthropus tchadensis.
Sahelanthropus tchadensis
It was followed by genuses Orrorin tugenensis, Ardipithecus, Australopithecus, Kenyanthropus, Paranthropus and finally our genus, Homo, showed up about 2.3 million years ago.
These Homo genus beings were not Homo sapiens, however, but Homo habilis, the earliest known member of our genus.
Homo habilis
These were not what we consider to be modern humans.
In between the emergence of Homo habilis and the eventual arrival of Homo sapiens (us), were numerous Homo species (such as rudolfensis, ergaster, erectus, antecessor, heidelbergensis, neanderthalensis, and more).
It was in south and east Africa, about 300,000 years ago, that modern humans, Homo sapiens (sensible humans), emerged.
Homo sapien
We remained there for about 200,000 years.
About 100,000 years ago, we began our migration north.
We arrived in what we now call Asia about 50,000 years ago.
Moving east, we arrived in what we now call Europe about 35,000 to 40,000 years ago.
Moving east and south in Asia, we arrived in what we now call Australia about 40,000 years ago.
Moving east in Asia across what we now call the Bering Strait about 12,000 to 15,000 years ago, we arrived in what now call the Americas about 12,000 years ago.
The family of humans began five to six million years ago as hunter-gatherers and remained so through all the genuses and homo species.
Hunter-Gatherers
About 10,000 to 12,000 years ago, with the domestication of plants and animals in various locations, our Agrarian Age began.
Agrarian Age
We remained in the Agrarian Age until the late 1700s, when the Industrial Revolution began in England.
By the mid-1800s, the Industrial Revolution spread to Belgium, Germany, France, and the United States.
Eventually, it spread to all the industrial nations.
In about the middle of the 1900s, we began our transition from the Industrial Age into a post-industrial high-technology age.
This led directly to the Information and Communication Age that emerged in the latter part of the century.
For the first time in our history, we were able to disseminate information almost anywhere instantly.
The Information Age has allowed rapid global communications and networking to shape modern society.
Our computing capabilities are growing exponentially.
It is likely that change will continue at an unprecedented rate, gaining momentum with time.
Everything, everywhere, will be connected.
It is estimated that we will be able to know anything, anytime, anywhere.