10.1: In the Beginning

Conditions for first life

Figure \(\PageIndex{1}\): Artist rendition of Hadean Earth. (Artist: Raymond R. Kilburn, Jr. Used with permission.)

No one can say exactly when and where life began on Earth, but it is certain that the origin of life is one of the most critical, planet-changing events ever to occur. Based on scientific observation and reasoning, theories have been developed to explain the ideal conditions that allowed life to start. Conventional scientific thought suggests that extreme heat from formational, early meteorite bombardment of Earth during the Hadean would have made conditions at the surface too harsh for life to begin–literally, portions of the early crust melted from these collisions (e.g., Marchi et al., 2014). However, despite continued impacts during the Late Heavy Bombardment, some studies that suggest only a fraction of the planet’s crust would have melted, leaving potential areas of refuge deep in the crust or within hydrothermal vents at the bottom of the oceans. Based on geochemical data, oceans are thought to have developed very early in Earth’s history, by about 4.3 billion years ago (i.e., during the early Hadean). Thus, while the possibility exists that life could have arisen during the Hadean, given the minimal availability of rocks from this interval, scientists in search of first life are typically working in rocks of Archean age.

Figure \(\PageIndex{2}\): Apex Chert of western Australia contains of of the oldest known fossils

Finding definitive evidence of the first life forms on Earth presents a challenge. While there are geochemical signatures to many early Archean rocks that suggest organisms may have been present, the organisms would be microscopic cells with no hard parts. Additionally, most of the Archean rocks that potentially preserve these fossils have been subjected to well over 3 billion years of tectonics and weathering. Schopf (1993) proposed they had discovered the oldest known fossils in the 3.5 billion year old (Archean) Apex Chert of western Australia. Debate quickly ensued in peer-reviewed literature, and many geoscientists now interpret the filamentous structures from the Apex Chert are the result of mineral growth around hydrothermal vents (e.g., Marshall et al., 2011). Currently, sulfur-metabolizing microfossils in the 3.4 billion year old (Archean) hydrothermal cherts of the Strelley Pool Formation of western Australia are considered the oldest definitive organic microfossils (Wacey et al., 2011; Alleon et al., 2018). However, Dodd et al. (2017) showed evidence of early life as tubular and filamentous structures near hydrothermal vent deposits of the 3.77-4.28 billion year old (Archean) rocks of the Nuvvuagittuq Belt in Quebec, Canada. While all of these findings continue to be examined in further detail, there is a common thread–early life seems to have begun in association with the warm, chemically enriched environments of hydrothermal vents.

Figure \(\PageIndex{3}\): Black smoker at hydrothermal vent along mid-ocean ridge in the Atlantic Ocean. (P. Rona / OAR/National Undersea Research Program (NURP); NOAA / Public domain)

Today, hydrothermal vents are found in the deep ocean along mid-oceanic ridges at divergent plate boundaries and at volcanic hot spots. In these environments, cold water from the bottom of the ocean seeps along fissures found at the plate boundary. While underground, the water is heated by and mixes with fluids released from the rising magma. The super-heated water is able to carry many dissolved minerals and mobile ions. These ions are carried along with the water to the seafloor, and are then precipitated as mineral deposits around the vents. Sometimes these mineral deposits are infused into the surrounding deep sea sediments, and sometimes they build impressive “chimneys” of towering rock. Sulfur-based minerals are particularly common near the vents. The vents are surprisingly diverse in life, with sulfur-reducing bacteria extracting energy from compounds in the vented fluids. Using the principle of uniformitarianism, the first organisms at Hadean hydrothermal vents were therefore also likely sulfur-reducing bacteria. These bacteria were chemoautotrophs, meaning they extract the energy necessary for cellular functions from inorganic compounds. In the case of sulfur-reducing bacteria, they “breathe” sulfate (\(\ce{SO_4^{2-}}\)), and reduce sulfur to hydrogen sulfide (\(\ce{H2S}\)). Sulfur-reducing bacteria are found today at the Mid-Atlantic Ridge, East Pacific Rise, and in pools associated with land-based hydrothermal activity like the hot springs and geysers of Yellowstone National Park in Wyoming. Some sulfur-reducing bacteria are also found in coastal environments–if you have ever noticed a rotten-egg odor in salt marshes, you’ve smelled the scent of chemoautotroph productivity! These prokaryotes belong to the Domain Archaea, which includes a diverse array of microbes that are able to live in chemically harsh conditions over a wide range of temperatures.

The prokaryotic pioneers

So the first life forms on Earth were likely sulfur-reducing bacteria, which are prokaryotes. Prokaryotes are structurally rather simple–they are single-celled, small in size (0.1 – 10.0 \(\mu\)m), and have no nucleus or other organelles. Make no mistake though, these simple organisms have changed the trajectory of Earth’s history in major ways. To quote Shakespeare, “Though she be but little, she is fierce!”

Figure \(\PageIndex{4}\): Phylogenetic tree of life showing the three major Domains. (Public Domain image.)

Two of the three Domains of life, Archaea and Bacteria, are prokaryotes. The third Domain, Eukarya, to which you belong, is discussed later in this chapter. Prokaryotes live on and in just about everything on Earth. They have been found in sulfuric volcanic lakes, hypersaline springs, glacial ice, growing on crystals deep in caves, and under rocks at the tops of mountains. In the human body, they outnumber human cells by 3 to 1. Rough estimates of modern oceanic prokaryotic diversity suggest more than 2 million species exist, which does not even include all the possible diversity on land. Consider how quickly bacteria evolve–check out this video that shows bacterial mutations against antibiotics in just 11 days! Then, remember that prokaryotes have been evolving for 3.5 billion years–there is no way the true diversity of prokaryotes will ever be known!

Figure \(\PageIndex{5}\): Stromatolite from the Archean Dresser Formation, Western Australia. (Creative Commons Attribution 2.0 Generic license.)

In many rocks of similar age as those preserving the Archean sulfur-reducing bacteria, macroscopic sedimentary structures known as stromatolites record the history of another important group–the cyanobacteria. Stromatolites are layered structures created when mats of cyanobacteria are smothered by fine calcite mud distributed by wave action in shallow marine environments. The bacteria grow through the mud layer to create a new mat, and the process repeats, producing a thinly laminated structure that is visible when lithified. Nutman et al. (2016) suggested they had found stromatolites in Greenland that were 3.7 billion years old (Archean), but this has been disputed by Allwood et al. (2018). The most widely accepted definitive stromatolites are those in the 3.5 billion year old (Archean) Dresser Formation in Australia.

Figure \(\PageIndex{6}\): Modern unicellular bacteria from a microbial mat in Guerrero Negro, Baja California, Mexico. (Public Domain via NASA.)

Cyanobacteria are photoautotrophs, meaning that they require sunlight to create their own food via photosynthesis. Therefore, these organisms are found in the shallow, photic zone of marine environments where sunlight can readily penetrate the water. Photosynthesis uses carbon dioxide (\(\ce{CO2}\)) as a source of carbon for cellular functions, and produces free oxygen (\(\ce{O2}\)) as a waste product. Cyanobacteria were the dominant life form on Earth for about 2 billion years, spanning the Archean and a major portion of the Proterozoic. As a result of all that photosynthesis, the amount of \(\ce{O2}\) in Earth’s atmosphere significantly increased over time in one of the most slow-building (but literally life-changing) events of all-time–the Great Oxidation Event. Banded iron formations (BIFs) are first preserved in the rock record around 2.4 billion years ago (Archean). BIFs consist of alternating layers of dark grey to black iron oxides (hematite and magnetite) with red to yellow chert.

Figure \(\PageIndex{7}\): Banded iron formation, Barberton Greenstone Belt, South Africa. (Creative Commons Attribution-Share Alike 1.0 Generic license.)

The interpretation of BIFs is that excess oxygen created by the photosynthesizing cyanobacteria in shallow oceans was available to combine with iron ions in seawater to produce the dark oxidized iron layers. The source of iron is likely a combination of both ions released from hydrothermal vents as well as input from chemical weathering of mafic-rich crustal rock from land. These distinct chemical sedimentary rocks were globally pervasive until about 1.8 billion years ago (Paleoproterozoic). BIFs decline in the rock record as the amount of oxygen in the atmosphere continued to climb. With more atmospheric oxygen available, iron ions were oxidized in terrestrial settings before they were transported to the oceans.

Eukaryotes: one small step for cells, one giant leap for life

A consistent pattern that we see in the rock record is that life modifies Earth systems, and Earth systems, in turn, modify life. The accumulation of oxygen in the atmosphere due to prokaryotic photosynthesis ultimately created a new resource that life could exploit as a source of energy. Around 1.8 billion years ago (Paleoproterozoic), definitive eukaryotes appear in the fossil record, armed with a new method of making energy for cellular function–aerobic respiration. Aerobic respiration requires oxygen to produce molecules of ATP which can then be stored and metabolized when needed to power a cell.

Figure \(\PageIndex{8}\): Prokaryotes versus eukaryotes. (Public Domain image; Science Primer (National Center for Biotechnology Information))

Eukaryotic cells contain a membrane-bound nucleus that houses the cell’s DNA and a variety of other organelles that assist with cell activities. In particular, mitochondria are important organelles because they are associated with ATP production within all eukaryotic cells. In plants, chloroplasts are also necessary as the sites where photosynthesis occurs within the cell. Because they contain a variety of organelles, eukaryotic cells are generally one to two orders of magnitude larger than prokaryotic cells (eukaryotes are typically 10 – 100 \(\mu\)m).

Figure \(\PageIndex{9}\): Schematic of endosymbiotic theory showing the evolution of eukaryotes from prokaryotes. (Image via Creative Commons Attribution-Share Alike 4.0 International license)

So, how did eukaryotic cells first evolve? Current science supports endosymbiosis theory (ET), which states that eukaryotic cells arose due to a mutually beneficial relationship where one prokaryote lives inside another. For example, one prokaryote engulfed another but did not digest it, allowing both cells to persist. The host cell offered protection, and the ingested cell provided energy to allow both cells to function. Over time, the cells co-evolved, and became reliant on each other as a single system: a cell with energy-producing organelles, mitochondria and chloroplasts. ET was first championed by Lynn Margulis in the 1960s, but took decades to be more widely accepted. One of the major lines of evidence in support of ET is that both mitochondria and chloroplasts possess their own DNA, with genetic signatures very similar to modern prokaryotes. Additionally, organisms without mitochondria, like the amoeba Pelomyxa, instead host symbiotic bacteria that serve the same function as mitochondria, but are not yet so wedded to their host that they cannot survive on their own.

Figure \(\PageIndex{10}\): Grypania spiralis fossils. (Creative Commons Attribution-Share Alike 3.0 Unported license.)

Today, eukaryotes include a wide variety of single-celled organisms, both with and without shells, like amoeba, foraminifera, coccolithophores, radiolaria, and diatoms. These are called protists. We classify them within the Kingdom Protista. All multicellular organisms are also eukaryotes, including all fungi, plants, and animals (Kingdoms Fungi, Plantae, and Animalia, respectively). The most widely accepted fossils that are interpreted as representing the first eukaryotes are acritarchs, a group of single-celled organisms that appear in the record about 1.8 billion years ago (Paleoproterozoic), well into the prokaryotic-driven Great Oxidation Event. Acritarchs are of unknown affinity, which means there is uncertainty as to where they fit on the tree of life. Acritarchs consist of a spherical sac-shaped cell ranging in size from 1.0 – 1000.0 \(\mu\)m, making them significantly larger than prokaryotes. The oldest forms are most commonly thought to be related to marine algae or dinoflagellates (a protist group). Also around 1.8 billion years ago (Paleoproterozoic), another fossil, Grypania spiralis, appears in the fossil record. This filamentous, spiral shaped fossil is commonly considered to be eukaryotic, primarily due to its size, leading some researchers adamantly debate that Grypania may also represent one of the first multicellular organisms on Earth.

Out onto the multicellular branches of life: the Ediacaran biota

Given that unicellular organisms had been quite successful for over a billion years, paleobiologists have wondered–what is the evolutionary benefit of a multicellular existence? Many scientists suggested that the typical pressures of natural selection, such as predation, could have played a role. In two interesting modern lab studies of unicellular green algae (Boraas et al., 1998; Herron et al., 2019), algal populations that grew into multicellular structures showed greater survival under predation. Multicellularity is likely an example of convergent evolution, occurring multiple times within different eukaryotic groups to create the diversity of multicellular eukaryotes that exist on Earth today.

Another potential evolutionary benefit shared by most multicellular organisms is the ability to reproduce sexually. Prokaryotes typically reproduce asexually by budding or fission, but the resulting organism is a genetic replicate unless a random mutation occurs. In the case of sexual reproduction, gametes (e.g., sperm and egg cells) recombine DNA to make genetically distinct offspring. Sexual reproduction does have certain disadvantages. For example, it takes a significantly longer time to reproduce and grow population sizes when only half the population (i.e., females) are capable of reproduction. However, the evolutionary trade-off from bringing together genes from two individuals can more rapidly produce advantageous traits and delete harmful mutations from a population. For example, the Red Queen hypothesis of sexual reproduction proposes that asexual organisms are more susceptible to parasitism due to their cloned DNA. Recombination of DNA through sex would allow an organism the genetic variability needed to defend against co-evolving parasites in the environment (Jokela et al., 2009). Given the incredible diversity of life on Earth, clearly both asexual and sexual reproduction are successful means to fully take advantages of Earth’s resources.

Figure \(\PageIndex{11}\): Artistic rendering of Ediacaran shallow seas. (Creative Commons Attribution-Share Alike 4.0 International license.)

As with other forms of early life, the origins of multicellular life are difficult to pinpoint given the lack of hard parts available for preservation. The fossil record of the Ediacaran Period (635-541 million years ago) at the end of Neoproterozoic best captures this next major innovation of life. The Ediacaran biota are a diverse assemblage of complex, macroscopic multicellular body and trace fossils. This significant increase in diversity follows an extensive period of glaciation (Snowball Earth) during the Cryogenian (Neoproterozoic). Prokaryotes likely contributed to eukaryotic success; this biota would have originated during a time when oxygen levels in the surface oceans were still increasing due to photosynthesizing cyanobacteria. Recent geochemical research (Bekker et al., 2017) also indicates bacteria levels were high in the Ediacaran seas, creating higher concentrations of dissolved organic matter as a food resource for these organisms. The warmer, oxygen- and food-rich seas of the Ediacaran may have set the conditions necessary for multicellular life to globally flourish.

Ediacaran fossils have been found on every continent except Antarctica. They are typically preserved within sandstones of shallow marine settings, like the famous deposits from the White Sea Coast of Russia, the Flinders Range of Australia, and Namibia. In a few places, however, fossils of the biota have been found in finer-grained sediments of deeper waters, such as the ash beds at Mistaken Point in southeastern Newfoundland. The biota primarily consists of impressions (both casts and external molds) of soft-bodied organisms that do not usually preserve well in coarser, sandy sediments, and many are even found underlying turbidity current deposits, effectively smothered by the rapid burial. Some research suggests that the fine texture of pervasive microbial mats would aid in preservation of the fossil impressions.

Figure \(\PageIndex{12}\): Dickinsonia tenuis fossil. (Creative Commons Attribution-Share Alike 4.0 International license.)

The fossils reveal a few common shapes (also known as morphologies). Many are described as disc-shaped (Cyclomedusa, Mawsonites). Others, known as rangeomorphs, have a frond-like shape (Rangea, Charnia). Still others exhibit the first clear examples of bilateral symmetry (Kimberella). There are even some odd forms that have an apparent trilateral symmetry (Tribrachidium), which is unlike anything alive today. The coarse sediment in which these fossils are preserved has hidden much of their anatomic detail, thus where these organisms should be placed on the tree of life has been widely debated. Some researchers suggest the Ediacaran biota represents basal members of some certain invertebrate animal groups that take off in diversity during the subsequent Cambrian Explosion, such as cnidarians and mollusks. Others have argued for identification of various Ediacaran forms as giant protists, algae, worms, fungi, or in a phylum of their own, the Vendozoa. Recent chemical analyses of Dickinsonia fossils showed evidence of cholesteroids, molecular fossils of the compound cholesterol, which is only found in animal cells (Bobrovskiy et al., 2018). When compared to molecular analyses of the surrounding sediments, only stigmasteroids, molecular fossils indicative of the green algae of the microbial mats was found. This study supports the interpretation that at least some of the Ediacaran biota are some of the first true animals on Earth.

While many ichnotaxa from this time have been re-evaluated in recent years to be sedimentary structures or microbially related, most Ediacaran burrows are simple, horizontal trails and burrows, or rasping traces made in association with microbial mats. Rasping is a mode of feeding by scratching at a surface. Snails and sea urchins eat algae this way today. Toward the end of the Ediacaran, burrows begin to increase in complexity, with a shift to more vertical and branching shapes, suggesting animal motility was on the rise.