Early Life

No part of the story of life holds more mystery or fascination than its ultimate origins. Cosmologists, geologists, paleontologists, and biologists have collected, compared, scrutinized, evaluated, and revised many kinds of evidence in order to see into the past. As a result, well accepted theories now illuminate nearly 4 billion years of life’s history, 4.6 billion years of Earth’s history, and even 13.7 billion years back to the Big Bang, which began the universe as we know it. Yet until the 19th century, most people believed the Earth was just 6,000 years old. We still do not know whether life exists beyond our Earth, nor can we predict where evolution will take life on Earth in the future, and our theories leave many chapters of the story untold. As you explore the early history of life, you must remember that the nature of science is to continue to question its own conclusions, to persist in seeking new information, and to readily modify or even overturn long-accepted theories, if new evidence contradicts them. Included is some of the best explanations science can currently provide for life’s origin and early evolution. A story of stardust, explosions, collisions, competition, and cooperation should not disappoint you, but it probably won’t give you all the answers you seek.

Formation of Earth: We are made of Stardust!

We will begin our story of the origin of life by exploring the origin of the materials which build it. The materials have a beauty and diversity of their own; perhaps your study of the Periodic Table of the Elements gave you an appreciation for their variety and individuality. Earth began as the solar system began – often described as a giant rotating cloud of dust, rocks, and gas. “Dust, rocks, and gas” may not sound inspiring, but this cloud contained the 92 elements or kinds of atoms which somehow combine to form every corner – living and nonliving - of the exquisite world we inhabit. The Big Bang (9 billion years earlier!) produced the atoms of hydrogen and helium. Elements as heavy as lithium followed the Big Bang within minutes. Stars such as red giants fused hydrogen and helium nuclei to form elements from carbon (the foundation of life!) to calcium (now our bones and teeth). Supernova explosions formed and ejected heavier elements such as iron (for red blood cells). We are not just “dust.” We - and our world - are stardust!

How did this rotating cloud of stardust become our solar system? One theory suggests that a nearby supernova sent a shock wave through the cloud, increasing its spin to form a proto-planetary disk, shown in Figure 1. Most of the mass concentrated in the middle and began to heat up, but large debris and collisions resulted in concentrations of matter outside the center. Eventually, heat in the central core began nuclear fusion of hydrogen to helium, and the Sun ignited. Matter outside the Sun’s gravity separated into rings of debris, and collisions of objects within the rings formed larger objects, which eventually became the planets. Solar wind cleared much of the remaining non-planetary material from the disk.

Figure 1: At left is an artist’s conception of the protoplanetary disk, which eventually formed our solar system. At right is an X-ray image of the remnant of Kepler’s Supernova, SN 1604, constructed of images from NASA telescopes and observatories. Together, art and science suggest the beauty of the “dust, gas, and rocks” which gave birth to our earth and its life.

One of the collections of debris, approximately 150 million kilometers from the Sun, was the protoplanet Earth. Newborn, Earth was very different from the home we know today. Bombarded by debris and heated by radioactive decay and the pressure of contraction, the Earth at first was molten. Heavy elements sank to the center, and lighter ones surfaced. Heat and solar wind meant that no atmosphere and no oceans were present.

Eventually, contraction and cooling allowed formation of a crust and retention of an atmosphere. However, continued bombardment melted portions of the crust for long periods. About 4.5 billion years ago, Earth collided with another protoplanet, Theia. This “big whack” gave us our moon and tilted Earth on its current axis, leading to the seasons, which now influence so much of life’s diversity. The Big Whack may also have initiated plate tectonic activity by speeding up the Earth’s rotation. Since then, however, the moon’s tidal drag may be slowing that rotation; scientists suggest that the day/night cycle during the Hadean may have been as short as 10 hours.

As the Earth continued to cool amidst heavy bombardment, steam escaped from the crust and active volcanoes released other gases to form a primitive atmosphere, which contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, but no more than a trace of oxygen. In the absence of oxygen, no ozone layer protected Earth from the Sun’s ultraviolet rays. Between 4.2 and 3.8 billion years ago, clouds and rain formed the oceans. The oceans were olive green, and the reddish atmosphere would have been toxic to modern multicellular organisms. Yet the stage was set for life to begin.

First Organic Molecules: Hypotheses About the Origin of Life's Chemistry

The Hadean Eon ended 3.8 billion years ago, its timeline marked by Earth’s oldest known rocks (between 3.8 and 4.2 billion years old) and oldest known minerals (formed 4.4 billion years ago). Scientists use these dates to estimate that the Earth itself is 4.6 billion years old. Evidence for life during the Hadean does not exist, although many scientists push the theoretical origin back that far. How – and when – did life arise?

Once again, we will begin with the materials of life – this time, organic molecules, made primarily of the element carbon. Most scientists agree that these organic molecules arose before cells, which we now consider essential to the definition of life. Several hypotheses and experiments suggest ways in which organic building blocks may have formed.

In 1924, Aleksandr Oparin proposed that life could have developed through gradual chemical evolution in a “primordial soup.” In 1953, Stanley Miller and Harold Urey designed a now-famous test of the hypothesis that the conditions of primitive Earth favored chemical reactions that synthesized organic molecules from inorganic precursors. Their experiment (Figure 2) showed that a mixture of gases, believed to be part of the primitive Earth atmosphere, when subjected to sparks representing lightning, formed a mixture of monomers representing each of the four major groups of organic molecules. Although DNA, RNA, and polymers were absent, 13 of the 22 amino acids that make up modern protein, plus lipids, sugars, and some building blocks of DNA and RNA, were among the products of the experiment.

Figure 2: The Miller-Urey experiment subjected a mixture of gases thought to be present in Earth’s primitive atmosphere to sparking, representing lightning. After one week, the nonliving system had formed 13 of the 22 amino acids which make up modern proteins, sugars, lipids, and some of the building blocks of DNA and RNA.

The “leap” from building blocks to polymers and from organic soup to individual replicating units has been more difficult to demonstrate. In the ‘50s and ‘60s, Sydney Fox showed that early Earth conditions could result in short chains of amino acids, which in turn could form enclosed spheres. Phospholipids can self-organize into membranes in a similar fashion, and cell membranes today consist primarily of a bi-layer of these lipids. Phospholipids or polypeptides could have surrounded and protected early metabolic units, forming protocells shown in Figure 3, simple membrane-enclosed spaces which may have led to the later evolution of true cells.

Figure 3: Phospholipids, with hydrophilic phosphate “heads” (P) and hydrophobic lipid “tails” (L) self-assemble into membranes (1) and enclosing spheres (2) which could have protected early metabolism from “outside” chemical disturbances.

Walter Gilbert, Carl Woese, and Alexander Rich proposed that RNA, because it can serve both catalytic and replicating functions, was the first informational molecule, and formed the “RNA World Hypothesis” for the origin of life. Sol Spiegelman created a short chain of RNA which was able to replicate itself in the presence of RNA polymerase; the segment is now known as the “Spiegelman monster.” The idea that a successful replicator molecule preceded the evolution of biochemical pathways is the “Genes-First” model.

In contrast, Günter Wächtershäuser proposed that sulfides of iron and other minerals contain energy which could have polymerized basic building blocks. He argued that extensive evolution of biochemical pathways might have preceded replicator molecules and individualization of life. His ideas formed the basis of William Martin and Michael Russell’s 2002 hypothesis that black smokers at seafloor spreading zones, shown in Figure 4, could have provided conditions for extensive chemical and biochemical pathway evolution. Their reasoning suggests that lipid membranes allowing independent lives away from the smokers could have been a last step in early evolution. The fact that archaebacteria and eubacteria (and us eukaryotes!) have completely different membrane lipids but similar metabolism supports the concept of early biochemical pathway evolution. These ideas comprise the “Metabolism-First” model.

Figure 4: Black smokers at a mid-ocean ridge hydrothermal vents could have provided conditions suitable for the evolution of early biochemical pathways and much of metabolism, even before lipid membranes formed cells. Martin and Russell propose that the last universal common ancestor may have emerged from a black smoker.

The discovery of organic molecules in space supports the exogenesis hypotheses which propose that life could have originated elsewhere – on Mars, or at some distant point in the universe. Comets and meteorites are known to contain organic molecules, and could have delivered them to Earth. Exogenesis does not really answer the question of how life originated, but provides a much wider temporal and spatial framework in which it could have happened.

Emergence of Life: The First Cells were Prokaryotes

Although many hypotheses and some experiments and observations explore the origin of cellular life, actual events remain unknown. If earth’s life first arose on earth, rather than by exogenesis, its timing is speculative, for no fossils record that event. Admitting that many conflicting hypotheses exist, we often express our current understanding of the story this way:

Perhaps four billion years ago during the Hadean Eon, lightning and a primitive atmosphere produced an organic soup of chemicals. As the “soup” became more concentrated, molecules began to interact with one another. As molecules became more complex, some molecules helped to speed up or catalyze chemical reactions (perhaps RNA, but eventually protein). Within that highly reactive soup, a molecule gained the ability to copy itself, becoming the first replicator (perhaps RNA, but eventually DNA). Copies contained errors, and errors which prevented replication caused the copies to “die out.” Copies that replicated faster survived to make more copies. Eventually, lipid membranes surrounded some of these chemicals, protecting them from reacting with other chemicals.

Although many protocell “species” probably populated the early “soup,” scientists believe that only one – a last universal common ancestor (LUCA) – emerged about 3.5 billion years ago during the Archean Eon, and later gave rise to all cellular life on earth. This prokaryote probably had a cell membrane and ribosomes, and used DNA for information storage, RNA for information transfer, and protein for catalyzing chemical reactions – like all life today. The first cells were probably heterotrophs, feeding on energy-rich chemicals concentrated in the “soup.” Alternatively, they could have been chemoautotrophs, extracting energy from inorganic molecules. Not long after prokaryotic cells emerged, they split into two major groups, Eubacteria and Archaebacteria. Both persist today, although Archaebacteria more often inhabit extreme habitats.

Inevitably, a diminishing supply of food molecules led to competition. At some point, glycolysis evolved as a pathway for transferring energy from organic molecules to ATP. This pathway persists in almost all organisms today.

Eventually, about three billion years ago, a new strategy evolved among some prokaryotes, which used sunlight to make carbohydrates from carbon dioxide and water. Photosynthesis provided a new source of food molecules for both autotrophs and the heterotrophs that “learned” to consume them. The oldest fossils, stromatolites, (Figure 5) record abundant photosynthetic cyanobacteria from that time.

Figure 5: Stromatolites are microbial mats made by some of the earliest photosynthetic organisms on Earth. Fossil stromatolites (left) are among the oldest fossils on Earth, although some have been interpreted to be of abiotic origin. Living stromatolites (right), mats of cyanobacteria, are found primarily in hypersaline lakes and marine lagoons.

Oxygen produced by photosynthesis first oxidized iron dissolved in the oceans, creating massive deposits of iron ore. Eventually, toward the end of the Archean, oxygen began to accumulate in the atmosphere, creating a major environmental change that is sometimes called the “Oxygen Catastrophe.” Oxygen was indeed toxic to many of the prokaryotes which had evolved as anaerobes. However, ultraviolet rays converted some of the oxygen to ozone, which prevented much of that harmful radiation from reaching the earth’s surface.

Thus, while an oxygen atmosphere may have killed many species, it allowed survivors to colonize previously uninhabitable ocean surface and terrestrial habitats. Even more important to the future of life, some prokaryote survivors “learned” how to use oxygen to harvest a great deal more energy from organic molecules. The energy efficiency of aerobic respiration paved the way for the emergence of larger and more complex organisms in the Proterozoic Eon.

Eukaryotes: Alliance, Invasion, or Slavery?

You have learned that our own eukaryotic cells protect DNA in chromosomes with a nuclear membrane, make ATP with mitochondria, move with flagella (in the case of sperm cells), and feed on cells which make our food with chloroplasts. All multicellular organisms and the unicellular Protists share this cellular intricacy. Bacterial (prokaryotic) cells are orders of magnitude smaller and have none of this complexity. What quantum leap in evolution created this vast chasm of difference?

Figure 6: The Endosymbiotic Theory holds that eukaryotic cells arose when larger prokaryotic cells engulfed smaller, specialized prokaryotes, without later digestion. The smaller cells reproduce independently within the larger cells, to the potential benefit of both. The diagram shows possible events leading to endosymbiosis. Black: membrane; Pink: eukaryotic DNA; Green: cyanobacteria/chloroplast DNA; Red: proteobacteria/mitochondrial DNA

The widely accepted Endosymbiotic Theory, shown in Figure 6, proposes that many organelles were once independently living cells. Larger cells engulfed these smaller cells but did not digest them, perhaps due to prey defenses. Alternatively, perhaps the smaller cells invaded the larger cells with the “intent” to parasitize. In either case, with their own DNA, the endosymbionts reproduced independently within the cell, and cell division passed them on to future generations of cells. Aerobic bacterial invaders would have been able to use oxygen to further break down and use energy from the host’s “wastes” from glycolysis. So much energy (ATP) resulted that some was available to the host; a mutually beneficial symbiosis resulted. This intriguing story of cooperation – so different from natural selection’s emphasis on competition – explains the origin of our mitochondria. A similar tale is told for chloroplasts; the benefit for a heterotrophic “host” is clear. Some scientists view cilia, flagella, peroxisomes, and even the cell nucleus as endosymbionts, but these ideas are less widely accepted.

What is the evidence for this maverick evolutionary pathway? Biochemistry and electron microscopy provide convincing support for the Endosymbiotic Theory. The mitochondria and chloroplasts which live within our eukaryotic cells share the following features with prokaryotic cells:

• Organelle DNA is short and circular – and sequences do not match DNA in the nucleus.
• Molecules that make up organelle membranes resemble those in prokaryotic membranes – and differ from those in eukaryotic membranes.
• Ribosomes in these organelles are similar to those of bacteria – and different from eukaryotic ribosomes.
• Reproduction is by binary fission – not mitosis.
• Biochemical pathways and structure show closer relationships to prokaryotes.
• Two or more membranes surround these organelles.

The “host” cell membrane and biochemistry are more similar to those of Archaebacteria, so scientists believe eukaryotes descended more directly from that major group (Figure 7). However, the standard evolutionary tree cannot accurately depict our ancestry, because the origin of the eukaryotes combines traditional descent from the Archaea with landmark cohabitation alliances forged with the Bacteria.

Figure 7: The three major domains of life had evolved by 1.5 billion years ago. Biochemical similarities show that we Eukaryotes share more recent common ancestors with the Archaea, but our organelles probably descended from Bacteria by Endosymbiosis.

The timing of this dramatic evolutionary event (more likely a series of events) is not clear. The oldest fossil clearly related to modern eukaryotes is a red alga dating back to 1.2 billion years ago. However, many scientists place the appearance of eukaryotic cells at about 2 billion years. Some time within Proterozoic Eon, then, all three major groups of life – Bacteria, Archaea, and Eukaryotes – became well established. Geologists hypothesize the oldest supercontinent, Columbia, between 1.8 and 1.5 years ago, as the backdrop for the further evolution of these three domains.

Eukaryotic cells, made possible by endosymbiosis, were powerful and efficient. That power and efficiency gave them the potential to evolve new ideas: multicellularity, cell specialization, and large size. They were the key to the spectacular diversity of animals, plants, and fungi which populate our world today. Nevertheless, as we close the history of early life, reflect once more on the remarkable but often unsung patterns and processes of early evolution. Our “size-ism” sets us up to wonder at plants and animals, and ignore bacteria. Our human senses cannot directly perceive the unimaginable variety of single cells, the architecture of organic molecules, or the intricacy of biochemical pathways. Let your study of early evolution give you a new perspective – a window into the beauty and diversity of unseen worlds – now and throughout Earth’s history. Apart from the innumerable mitochondria which call your 100 trillion cells home, your body contains more bacterial cells than human cells. You, mitochondria, and your resident bacteria share common ancestry – a continuous history of the gift of life.