Life abounds on planet Earth. We are familiar with numerous forms, like people, pets, insects, and fish. Using microscopes, we see a host of bacterial and viral organisms. Digging in the dirt reveals bones of enormous dinosaurs. Life takes many different shapes, sizes, and lifetimes. Scientists find life in virtually every environment where they suspect life could live, and even in many they once thought life impossible. Bacteria thrive in boiling water, bubbling tar, extremely dry deserts, frozen glaciers, rocks two miles below Earth’s surface, and even in environments with radiation that would destroy cockroaches. The ubiquity of life on Earth can make it seem like life should also abound in the universe.
Over the past few decades, scientists have realized that both Earth and the universe underwent significant changes to permit life’s existence on Earth. In the beginning, the universe could not host life. When Earth first formed, it was also hostile to life. To evaluate the possibility of life “out there,” it is useful to remember what transpired from the beginning of the universe until today. This post provides an overview that briefly describes the important, life-critical transitions that occurred since the creation of the universe.
The Creation of the Universe
Fourteen billion years ago (13.8 billion, to be specific), our universe began. During the earliest moments of the universe, it expanded at a tremendous rate. This period of inflation ended with two important consequences. First, the total amount of stuff that astronomers can see (around 100 billion trillion stars spanning a sphere roughly 46 billion light-years in radius) constitutes just a small fraction of the amount of stuff out there. Second, as this epoch of inflation ended, it released an enormous amount of energy that heated the universe to unfathomable temperatures. As the universe cooled from this hot, dense state, a number of transitions that are important to the discussion of extraterrestrial life occurred.
The First Few Minutes
During the first fractions of a second, the quantum gravitational force governing all of the interactions in the universe separated into the distinct forces we see at work today—the gravitational, strong nuclear, electromagnetic, and weak nuclear forces. The gravitational force, operating among things with mass, affects how the universe expands, the sizes and lifetimes of stars, and the atmospheres of planets.
The strong nuclear force keeps the protons and neutrons bound inside a nucleus. The strength of this force influences the amount of various elements of the universe in two ways. First, during the first few minutes of the universe, temperatures and densities are high enough for hydrogen (the lightest element) to fuse into heavier elements. Second, stars also experience conditions where lighter elements (like carbon) fuse into heavier ones (like oxygen).
The electromagnetic force affects anything with charge as well as all forms of light. Consequently, it affects the sizes and lifetimes of stars as well as every chemical interaction necessary for life.
The weak nuclear force determines how heavier elements decay into lighter elements. It plays a critical role in how stars fuse hydrogen into helium as well as in the generation of heat in Earth’s interior (radioactive decay produces lots of heat).
At the end of four minutes, the universe had cooled significantly—its temperature had dropped below 1 billion degrees. Enough hydrogen had fused into helium that these two elements composed 75 percent and 25 percent of all the nuclei in the universe. Only trace amounts of beryllium and boron existed.
The Cosmic Microwave Background Radiation
Not much changed over the next 400,000 years, except that the universe continued to cool. Around 380,000 years, the temperature fell low enough that the hydrogen and helium nuclei could combine with the surrounding electrons to form atoms. As the atoms formed, they emitted a specific distribution of light that we now measure as the cosmic microwave background radiation. Scientists use this light to determine the age, mass density, expansion rate, size, and many other important characteristics of the universe.
The First and Later Generations of Stars
Over the next billion years, a couple of important changes occurred. The most significant one happened around 200 million years when stars started to form. For the first time since the early minutes of the universe, conditions for producing elements heavier than hydrogen and helium existed. These original stars contained hundreds of times more mass than that of the sun. Consequently, they burned their nuclear fuel very quickly (a few million years), exploded in dramatic supernova events, and scattered copious amounts of elements as heavy as uranium, neptunium, and plutonium into the material that would form future stars.
During the next few hundred million years, galaxies started to form as well. Later generations of stars formed from the ashes of the first stars. This continued to enrich galaxies with the heavier elements throughout the three generations of stars that astronomers have identified. While gas giant planets can form around stars with lesser amounts of heavy elements, only this third generation of stars had sufficient quantities of carbon, oxygen, uranium, and plutonium to make planets capable of supporting life.
The Formation of the Sun and Planets
Four and a half billion years ago, a supernova explosion sent a shock wave into a cloud of gas. This shock wave led to the collapse of the cloud, making the sun and planets in the process. For 5 to 10 million years, the planets grew by gathering gas, dust, and ice until the wind emitted by the young Sun drove all the planet growth material from the solar system. Two critical events happened over the next 500 million years. First, around 100 million years, a large, Mars-sized object collided with Earth. This impact event increased Earth’s mass, added radioactive elements to Earth’s interior, and most importantly, made our Moon. Second, during this 500-million-year period, Jupiter, Saturn, Uranus, and Neptune migrated from the places where they formed to their current locations. This period of migration moved the gas giants farther from the sun and likely caused Neptune and Uranus to switch positions.
Life’s Development on Earth
Earth’s original atmosphere contained no free oxygen, little (if any) land rose above the oceans, and large objects frequently collided with the planet. In spite of this rather hostile environment, evidence for life on Earth dates back to almost 4 billion years ago. Scientists have found fossilized life from 3.5 billion years ago and chemical evidence from another 300 million years earlier. Admittedly, this life is simple by today’s standards.
As mentioned above, the orbits of the gas giants changed significantly. The rate of comets and asteroids impacting Earth (as well as Mars, the moon, Mercury, and Venus) dramatically increased as the gas giant planets migrated to their current positions. Many of these impact events probably sterilized Earth’s surface, but the period of bombardment cleared the solar system of debris. Subsequently, the frequency of objects colliding with Earth decreased by a factor of 1,000. The evidence indicates that life appeared on Earth in abundance shortly after this “late heavy bombardment” (scientists’ designation for this period).
Over the next 2 to 3 billion years, the amount of land covering Earth’s surface increased. As scientists dated the formation of continental rocks, they discovered that the rocks clustered around a few ages—specifically, 1.2, 1.9, 2.7, and 3.3 billion years ago. The growth of continental land mass provided new environments for life to thrive as well as a thermostat that regulates Earth’s temperature. This thermostat function featured prominently when photosynthetic organisms started producing enough free oxygen so that Earth’s oceans and atmosphere began to contain a permanent oxygen component, roughly 2.5 billion years ago.
The Cambrian Explosion
One of the most dramatic changes in the history of life on Earth occurred about 540 million years ago. During a geologically short period of time, a wealth of multicellular organisms showed up in the fossil record in an event referred to as the “Cambrian explosion.” Before this time, the fossil record shows only the presence of single-celled life that occasionally organized into colonies. While animal life has changed significantly over the last 540 million years, almost all the different body plans (distinguished by different phyla) show up during the Cambrian explosion.
Humanity, the most unique form of life ever seen on Earth, arrived much more recently. Fossil, genetic, and archaeological evidence indicate that human beings started living about 100,000 years ago. While other animals share physiological features with humans, we are the only creatures that have a deep-seated capacity to relate to one another and an awareness of our own existence. One way this awareness manifests itself is the universal sense that God exists and that we must figure out how to properly relate to him.
A Match with Genesis
Even this brief description demonstrates a correspondence between our best scientific understanding of Earth’s history and the creation account given in Genesis 1. Both begin with the origin of the universe (the big bang, see Genesis 1:1) before moving to the initial stages of planet Earth, which is hostile to life at this time (Genesis 1:2). The bombardment period of Earth’s early history transformed the atmosphere so that light reached the planet’s surface (Genesis 1:3–5, the first day brings the day-night cycle to the surface). It also brought the water that is so critical to a stable water cycle (Genesis 1:6–8). In the middle of Earth’s history (Genesis 1:9–13, day three of six), most of the continents formed, which would also allow plants to grow. Complex, multicellular life appeared explosively during the Cambrian explosion (mirroring day five in Genesis 1:20–23). And humanity arrives very recently (at the end of day six, Genesis 1:24–31). Clearly, scientists have learned far more detail than the overview given in Genesis 1. However, it is remarkable that a book authored thousands of years before humanity had a thriving scientific enterprise gets the important details of Earth’s history correct!