The Earliest Life
And the roots of life on Earth become clear.
In 1859, in On the Origin of Species, Darwin repeatedly pointed to what he viewed as the greatest challenge facing his theory of evolution – the lack of a rich fossil record predating the rise of shell covered invertebrate animals at the beginning of the Cambrian Period (~550 million years ago).
Figure 1. Painted before his first voyage.
For more than 100 years, the missing Precambrian history of life stood out as one of the great unsolved mysteries in natural science.
However, this has changed, and the documented fossil record has been extended to 3,500 million years ago, more than three-quarters the age of our planet.
So, let's look at this record, starting with bacteria.
Figure 2. From the Bitter Springs chert of central Australia,
a site dating to the Late Proterozoic, about 850 million years old.
It may seem surprising that bacteria can leave fossils at all, but a particular group of bacteria – the cyanobacteria or "blue-green algae" -- have left a fossil record that extends into the Precambrian - nearly 3.5 billion years old, and are among the oldest fossils currently known.
But what does this really mean?
Side Trip – Geologic Time & Early Life
Without getting too deep in this, let's look at the Geologic Time Scale, so we have a better understanding of the range of time and the history of what was alive in those ages.
Figure 3.
Note Prokaryotes (magenta band) go back 4 Ga (4 Billion) at the end of the Hadean. At 3.5 Ma, Photosynthesis starts in the Archean (pronounced /ɑrˈkiːən/), and at half of the age of our Earth, 1.2 million years later, the atmosphere becomes oxygen-rich.
Science divides prokaryotes [ proh-kar-ee-oht (pro carry oat) ]into two domains: the bacteria and the archaea.
But with a similar structure. Here's what they look like.
Figure 4.
These early bacterial organisms lacked a cell nucleus, or any other membrane-bound organelles. But both, Bacteria and Achaea are major biological groups, classified as domains.
Here's an illustration that shows the biological classification system.
Figure 5.
So, bacteria and archaea are major domains of life. As you can see in Figure 3, these domains of life started extremely early in the history of the Earth, and provided the platform that we evolved from.
Note Modern Archaea have been found in a variety
of extreme environments, such as salt marshes, sewer outfalls, and
the hot springs in Yellowstone National Park. In fact, Archaea are
now considered so numerous that, one source estimates, the domain
may form up to half of Earth's biomass.
They are the closest relative now known to the first life on our
planet.
Here's the relationship between Bacteria and Archaea:
Figure 6.
As you can see, the Eucaryota [ yoo-kar-ee-oht (you carry oat) ] Domain came from Archaea, and from that line, animals ( you and I ), developed from them. Look back to Figure 3, the Geologic Time Chart, and notice when Eukaryotes began.
An eukaryote is an organism whose cells contain complex structures inside its membranes.
Figure 7.
The origin of the eukaryotic cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms.
And from current research, biomarkers suggest that at least stem eukaryotes arose even earlier with the presence of steranes in Australian shales indicating that eukaryotes were present 2.7 billion years ago.
Quick Review
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Our Earth is old -- 4.5 billion years.
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Research indicates life started 4000 Million years ago (Ma or Mya).
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Bacteria started making oxygen 3500 Mya.
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Bacteria and Achaea are major biological Domains, but they are simple organisms that don't have a cell nucleus.
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Eukaryotes developed from Achaea and contain complex structures inside its membranes.
The Early Fossils
No one knows exactly when life first appeared on Earth. According to some researchers, the oldest fossil microorganisms are as old as the oldest sedimentary rock.
So, we assume that life has been around as long as conditions have been suitable. But, suitable is not oxygen based. At the time of these first organisms, there was probably no free oxygen, as there is now.
At that time, scientists name the atmosphere as a "reducing atmosphere." This means it was composed of methane, carbon dioxide, hydrogen and water vapor.
During this period, the microorganisms may have used methane or hydrogen rather than oxygen in their metabolism. As a result, they are referred to as "anaerobic" (non-oxygen-using). Fermentation is modern example of anaerobic metabolism.
Here's a picture of modern archaea.
Figure 8.
Methanococcos jannaschii is a unicellular, microscopic creature
that looks like a squid but, with its spinning flagella, is only
6-millionths of a meter long. It thrives in a deep-sea vent environment
of high heat and pressure. It consumes hydrogen and carbon dioxide
for energy, nitrogen and phosphorus to make cell material, and transition
metals, such as iron, nickel, manganese and selenium, for cellular
functions and proteins. It produces natural methane.
So, simple microorganisms were around for millions of years, living in colonies, but did they leave any fossils and have they been found?
Here's a passage from Benchley and Harper, Palaeoecology, p.121
The … “early” (Pre-Vendian)... fossil record is dominated by
stromatolites, sheets of calcium carbonate associated with cyanobacteria.
Carbonate material is trapped and precipitated on the surface of the
filamentous bacteria to generate a distinctive laminated structure.
Stromatolites are generally rare in Archaean rocks, becoming more
common during the late Archaean and early Proterozoic.
Here's what modern stromatolites look like.
Figure 9. Hamelin Pool, Shark Bay, Australia.
A stromatolite mound is a solid structure created by the single-celled microbes. The cyanobacteria form colonies and trap sediment with their sticky surface coatings. The trapped sediment reacts to calcium carbonate in the water to form limestone.
These limestone deposits build up very slowly – it can take a stromatolite 100 years to grow 5 cm, so a one meter-high stromatolite might be 2,000 years old.
When these mounds are cut through, they look like this:
Figure 10. Stromatolites, formed from layers of blue-green
algae.
What Happened Next?
Stromatolite reefs formed on the shallow submerged wave-cut platforms around ocean volcanoes in the Archaean era, and on shallow continental shelves in the Proterozoic.
The turbid water overlying these reefs, and the silt and quartz built into the reef, offered protection from solar ultraviolet light. But the reef had to keep growing upwards so that the photosynthetic bacteria were not shaded by too thick a layer of silt, or submerged too deeply as the volcanic islands subsided towards the ocean floor.
Over hundreds of millions of years, the growth of stromatolites released oxygen (taken from water during photosynthesis).
At first this oxygen was used by reducing compounds, but rocks from the Proterozoic era suggest that free oxygen occurred in the atmosphere and ocean during this era, at first intermittently, then, later continuously.
The consequences of increasing oxygen levels were:
- The formation of eukaryote cells by symbiosis of bacteria, including oxygen-consuming bacteria that became mitochondria, and the photosynthesising bacteria that became the chloroplasts of algae and plants (Margulis, 1993);
Figure 11. Fossils of 2.1Mya Algae, Michigan, US.
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The evolution of multicellular animals which became predators on other animals and on micro-organisms;
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The formation of an ozone layer in the atmosphere, shielding the Earth's surface from harmful UV light and allowing life to move onto land.
Many of these topics are summarized in this diagram, showing the main events and changes in the Earth's history, from the beginning to now:
Figure 12.
Another illustration expands on the ideas, using the geologic time scale as a matrix in which key events in life's evolution are stated:
Figure 13.
Quick Review
A Microfossil Gallery
These are algae microfossils from the Precambrian, Proterozoic Eon.
An Algae cyst:
A multicelled algae fossil:
Terms
Anaerobic. Able to live where there is no free oxygen or air. Obligate anaerobes are unable to live in even small concentrations of oxygen, while facilitative anaerobes can live in low or normal concentrations of oxygen as well as where there is none.
Archaea. (pronounced "are-kay"). One-celled, microscopic organisms without a nucleus. Half to two-thirds of their genes are unlike anything else on earth.; they are ancient, first appearing 3 to 4 billion years ago, and tend to favor extreme environments such as hydrothermal vents.
Biomass. The total sum of plants and animals in a given area at a given time, expressed as weight of organism per unit area or as the volume per unit volume.
Links
Our Mysterious Ancient Reefs
by Jon Nelson
www.lakesuperior.com
Here's how to pronounce archaea:
upload.wikimedia.org
Here's How Microbial Mats Work (How Stromatolites work):
nai.arc.nasa.gov
Here's a series of eleven videos posted on Youtube about “How Life Began.”
www.youtube.com
Here's a video about Earth Science:
paleogeology.blogspot.com/
Here's the Stromatolite Identification Site:
www.evolutionaryresearch.org
Want to see an underwater video of stromatolites?
www.sharkbay.org
Figures & Acknowledgments
We want to thank all of the wonderful sites which helped illustrate this discussion, and we wish that all of you visit them as part of your reading.
We are humbled by the intelligence and grace of our science communities.
Figures
Figure 1.taggart.glg.msu.edu
Figure 2. www.ucmp.berkeley.edu
Figure 3. en.wikipedia.org
Figure 4. http://en.wikipedia.org/wiki/Prokaryote
Figure 5. en.wikipedia.org
Figure 6. en.wikipedia.org
Figure 7. www.hartnell.edu
Figure 8. library.thinkquest.org
Figure 9. sepmstrata.org
Figure 10. en.wikipedia.org
Figure 11. evolution.berkeley.edu
Figure 12. rst.gsfc.nasa.gov
Figure 13. rst.gsfc.nasa.gov
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