7.4: Evidence for Early Life

Evidence for Early Life

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Learning Objectives

• Discuss the origins of prokaryotic organisms in terms of the geologic timeline
• Distinguish Bacteria from Archaea in terms of their origins
• Discuss the distinguishing features of extremophiles and the environments that produce biofilms

Prokaryotes, the First Inhabitants of Earth

Video

This 5.5-minute video describes the search for the earliest evidence of life on earth.
Question after watching: Evidence for life on earth is that it happened relatively quickly after the formation of the planet. What are the implications of this hypothesis?

When and where did life begin? What were the conditions on earth when life began? Prokaryotes were the first forms of life on earth, existing for billions of years before plants and animals appeared. The earth and its moon are about 4.54 Ga. This estimate is based on evidence from radiometric dating of meteorite material together with other substrate material from the Earth and the moon.

Early earth had a very different atmosphere than it does today, for example, it contained less molecular oxygen since photosynthesis had not yet evolved to produce it as a by-product. It was also subjected to strong radiation from outer space since the composition of the early atmosphere did not protect the surface as it does today. Thus, the first organisms would have flourished where they were more protected, such as in ocean depths or beneath the surface of the earth. Also at this time, strong volcanic activity was common on Earth. It is probable that these first organisms, the first prokaryotes, were adapted to very high temperatures. The early earth was prone to geological upheaval, volcanic eruptions, and bombardment by mutagenic radiation from the sun. The first organisms were prokaryotes that could withstand these harsh conditions.

Although prokaryotic cell fossils date to almost 3.5 billion years ago, most prokaryotes do not have distinctive morphologies; fossil shapes cannot be used to distinguish them between Archaea and Bacteria. Instead, chemical fossils of unique lipids are more informative because such compounds do not occur in other organisms. Some publications suggest that archaean or eukaryotic lipid remains are present in rock formations dating from 2.7 billion years ago in Precambrian formations. The oldest such traces come from Greenland, in Earth’s oldest sediments, which formed 3.8 billion years ago. 

Optional Activity \(\PageIndex{1}\)

Why do scientists believe that the first organisms on Earth were extremophiles?

Answer

Because the environmental conditions on early Earth were extreme by current standards: high temperatures, lack of oxygen, high radiation, etc.

Current Model

The first, living organisms were likely prokaryotes, which may have evolved from one or more of the circumstances in Section 3.3.2. The evidence strongly suggests that eukaryotes evolved much later in the history of life. They are discussed in Section 3.3.4.

Evolution of Prokaryotes

In the recent past, scientists grouped living things into five kingdoms (animals, plants, fungi, protists, and prokaryotes) based on several criteria such as: the absence or presence of a nucleus and other membrane-bound organelles, the absence or presence of cell walls, multicellularity, etc. In the late 20^th century, the pioneering work of Carl Woese and others compared sequences of the small-subunit ribosomal RNA (SSU rRNA), which are found in every living organism. This resulted in insights into the evolution of living organisms, their relationships, and a more fundamental way to group organisms on earth. Based on differences in the structure of cell membranes and in the sequences of their rRNA, Woese and his colleagues proposed that all life on earth evolved along three domains. The Domain Bacteria comprises all eubacteria (a type of prokaryote), the Domain Archaea comprises the archaeans (also a type of prokaryote), and the Domain Eukarya comprises all eukaryotes, including organisms in the kingdoms Animalia, Plantae, Fungi, and Protista.

Two of the three domains, Bacteria and Archaea, are prokaryotic. Based on fossil and molecular evidence, prokaryotes were the first inhabitants on Earth, appearing 3.5-3.8 Ga during the Precambrian Period. These organisms are abundant and ubiquitous; that is, they are present everywhere on Earth today. In addition to inhabiting moderate environments, they are found in extreme conditions: from boiling hot springs to permanently frozen environments such as tundra and sea ice; from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen, such as a waste management plant, to radioactively-contaminated regions, such as Chernobyl in Ukraine. Prokaryotes reside in the human digestive system and on human skin, are responsible for some illnesses, and serve an important role in the preparation of many foods.

Video

This 4-minute video explains how bacteria almost extinguished life on earth early in life's history.
Question after watching: Why was the creation of oxygen by photosynthesis problematic?

Prokaryotic Life

Microbial Mats

Microbial mats, or large biofilms, may represent the earliest forms of life on earth; there is fossil evidence of their presence starting about 3.5 billion years ago. A microbial mat is a multi-layered sheet of prokaryotes that includes mostly bacteria, but also archaea (Figure \(\PageIndex{1}\)). Microbial mats are a few centimeters thick, typically growing where different types of materials interface, mostly on moist surfaces. The different species of prokaryotes that comprise the mats use different metabolic pathways, which is the reason for their varied colors. Prokaryotes in a microbial mat are held together by a glue-like sticky substance that they secrete called extracellular matrix.

Figure \(\PageIndex{1}\): (a) This microbial mat, about one meter in diameter, grows over a hydrothermal vent in the Pacific Ocean in a region known as the “Pacific Ring of Fire.” The mat helps retain microbial nutrients. Chimneys, such as the one indicated by the arrow, allow gases to escape. (b) In this micrograph, individual bacteria in a microbial mat are visualized using fluorescence microscopy.

The organisms in the first biofilms likely obtained their energy from chemicals found near hydrothermal vents. A hydrothermal vent is a fissure in the Earth’s surface that releases geothermally-heated water as well as chemicals leached from rocks in the earth’s interior. With the evolution of photosynthesis about 3 billion years ago, some prokaryotes in microbial mats came to use a more widely available energy source, sunlight, whereas others were still dependent on chemicals from hydrothermal vents for energy and food.

Optional Activity \(\PageIndex{2}\)

Microbial mats __________.

1. are the earliest forms of life on Earth
2. obtain their energy and food from hydrothermal vents
3. are multi-layered sheets of prokaryotes including mostly Bacteria but also Archaea
4. all of the above

Answer

D. All of the above

The Ecology of Biofilms

Until a couple of decades ago, microbiologists used to think of prokaryotes as isolated entities living apart. This model, however, does not reflect the now better-understood ecology of prokaryotes, most of which prefer to live in communities where they can interact. A biofilm is a microbial community held together in a gummy-textured matrix that consists primarily of polysaccharides secreted by the organisms, together with some proteins and nucleic acids (Figure \(\PageIndex{2}\)). Biofilms grow attached to surfaces to secure them in place. Some of the best-studied biofilms are composed of prokaryotes, although fungal biofilms have also been described, as well as some composed of a mixture of fungi and bacteria.

Figure \(\PageIndex{2}\): Micrographs of a Pseudomonas aeruginosa biofilm in each of the stages of development. Biofilm development occurs in five stages as shown. During stage 1, initial attachment, prokaryotes adhere to a solid surface. During stage 2, irreversible attachment, hairlike appendages called pili permanently anchor the prokaryotes to the surface. During stage 3, maturation I, the biofilm grows through cell division and recruitment of other prokaryotes. An extracellular matrix composed primarily of polysaccharides holds the biofilm together. During stage 4, maturation II, the biofilm continues to grow and takes on a more complex shape. During stage 5, dispersal, the biofilm matrix is partly broken down, allowing some prokaryotes to escape and colonize another surface. 

Biofilms are present almost everywhere: they are visible on the surface of some lakes and can cause the clogging of pipes. They also colonize household surfaces, such as kitchen counters, cutting boards, sinks, and toilets, as well as places on the human body, such as the surfaces of our teeth (plaque). They also play a major role in food contamination.

Interactions among the organisms that populate a biofilm, together with their protective extracellular matrix, make these communities more robust than free-living prokaryotes. The sticky substance that holds bacteria together also excludes most antibiotics and disinfectants, making biofilm bacteria hardier than their free-living counterparts. Overall, biofilms are very difficult to destroy because they are resistant to many common forms of sterilization.

Stromatolites

Fossilized microbial mats represent the earliest record of life on earth. A stromatolite is a sedimentary structure formed when minerals are precipitated out of water by prokaryotes in a microbial mat (Figure \(\PageIndex{3}\)). Stromatolites form layered rocks made of carbonate or silicate. Although most stromatolites are artifacts from the past, there are places on earth where stromatolites are still forming. For example, growing stromatolites have been found in the Anza-Borrego Desert State Park in San Diego County, California. 

Figure \(\PageIndex{3}\): Stromatolites: (a) These living stromatolites are located in Shark Bay, Australia. (b) These fossilized stromatolites, found in Glacier National Park, Montana, are nearly 1.5 billion years old.

In addition, NASA is studying Pavilion Lake, north of Whistler BC, for its microbialites population (Figure \(\PageIndex{4}\)). These bacteria colonies live at the bottom of the lake and precipitate minerals that form coral-like structures. These structures formed 10,000 years ago, when the glaciers retreated from the area.    

Figure \(\PageIndex{4}\): Examples of microbialite morphologies in Pavillion Lake (CC-BY-SA; Schulze-Makuch, D. et al. https://www.mdpi.com/2075-1729/3/1/21#)

Video

This 2-minute video describes what stromatolites are.
Question after watching: Why are stromatolites relevant to the study of the origin and early evolution of life?

Microbes Are Adaptable: Life in Moderate and Extreme Environments

Some organisms have developed strategies that allow them to survive harsh conditions. Prokaryotes thrive in a vast array of environments; some grow in conditions that would seem very normal to us, whereas others are able to thrive and grow under conditions that would kill a plant or animal. Almost all prokaryotes have a cell wall: a protective structure that allows them to survive in both hyper- and hypo-osmotic conditions. Some soil bacteria can form endospores that resist heat and drought, thereby allowing the organism to survive until favorable conditions recur. These adaptations, along with others, have made bacteria the most abundant life form in all terrestrial and aquatic ecosystems today.

Other bacteria and archaea are adapted to grow under extreme conditions and are called extremophiles, meaning “lovers of extremes”. Extremophiles have been found in all kinds of environments: the depth of the oceans, hot springs, the Arctic and the Antarctic, in very dry places, deep inside the Earth, in harsh chemical environments, and in high radiation environments, just to mention a few (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Certain prokaryotes can live in extreme environments such as the Morning Glory pool, a hot spring in Yellowstone National Park, Wyoming, USA. The spring’s vivid blue color is from prokaryotes that thrive in its hot waters.

These organisms give us a better understanding of prokaryotic diversity and help us understand the range of environmental conditions that life can tolerate. This allows us to understand where life may have originated, and where we may find it in the future (including on other planets and moons in the solar system). In addition, the unique enzymes and chemical pathways that extremophiles develop to adapt to their environment raise the possibility of finding new therapeutic drugs or industrial applications.

Because they have specialized adaptations that allow them to live in extreme conditions, many extremophiles cannot survive in moderate environments. There are many different groups of extremophiles. They are identified based on the conditions in which they grow best. For examples, organisms that live in acidic (low pH) conditions are called acidophiles. Those that live in hot environments are called thermophiles. Some habitats are extreme in multiple ways. For example, a soda lake is both salty and alkaline, so organisms that live in a soda lake must be both alkaliphiles and halophiles.

Other extremophiles, like radioresistant organisms, do not prefer an extreme environment (in this case, one with high levels of radiation), but have adapted to survive in it (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Deinococcus radiodurans, visualized in this false-color transmission electron micrograph, is a prokaryote that can tolerate very high doses of ionizing radiation. It has evolved DNA repair mechanisms, refined from existing ones found in other organisms, but specialized for its environment, that allow it to reconstruct its chromosome even if it has been broken into hundreds of pieces by radiation or heat.

Prokaryotes in the Dead Sea

One example of a harsh environment is the Dead Sea, a hypersaline basin that is located between Jordan and Israel. Hypersaline environments are concentrated seawater. In the Dead Sea, the sodium concentration is 10 times higher than that of seawater. The water also contains high levels of magnesium (about 40 times higher than seawater) that are toxic to most living things. Iron, calcium, and magnesium, elements that form divalent ions (Fe²⁺, Ca²⁺, and Mg²⁺), produce what is commonly referred to as “hard” water. Taken together, the high concentration of divalent cations, the acidic pH (6.0), and the intense solar radiation flux make the Dead Sea a unique, and hostile, ecosystem.

Figure \(\PageIndex{6}\): (a) The Dead Sea is hypersaline. Nevertheless, salt-tolerant bacteria thrive in this sea. (b) These halobacteria cells can form salt-tolerant bacterial mats.

Career Connection: Paleontologist

Natural history museums display the fossil casts of extinct animals and information about how these animals evolved, lived, and died. Paleontologists are scientists who study past life. They use fossils to observe and explain how life evolved on Earth and how species interacted with each other and with the environment. A paleontologist must be knowledgeable in biology, ecology, chemistry, geology, and other scientific disciplines. A paleontologist’s work may involve field studies: searching for and studying fossils. In addition to digging for and finding fossils, paleontologists also prepare fossils for further study and analysis. Although dinosaurs are probably the first animals that come to mind when thinking about paleontology, paleontologists study everything from plant life, fungi, and fish to sea animals and birds.

An undergraduate degree in earth science or biology is a good place to start toward the career path of becoming a paleontologist. Most often, a graduate degree is necessary. Additionally, work experience in a museum or in a paleontology lab is useful.

Optional Resource

Paleontology in Western Canada

In Western Canada, there are several sites rich in fossils that are active sites of research. The Burgess Shales, located in Yoho National Park in BC (near Golden) is a designated World Heritage Site. Fossils from this location date to 508 Ma, in the middle of the Cambrian Explosion, when many animal body plans first evolved. Although the site is now at the top of the Rockies, 508 Ma it was at the bottom of a shallow sea where a mudslide likely buried many animals, preserving the structure of even soft-bodied creatures.

In Alberta, Dinosaur Provincial Park is also a UNESCO World Heritage site. Located 48 km east of Calgary, it is not just a recreational area but also an active research site for dinosaurs that lived roughly 75 Ma in the late Cretaceous. Tyrannosaurus, one of the most well-known large dinosaurs, are found here. Dinosaur bones are so plentiful that they are regularly seen sticking out of the ground.  Other active dig sites exist in Edmonton, Alberta’s river valley and around Grande Prairie.