The Evolution of Complex Life on Earth

How did complex life on Earth evolve? This incredibly beautiful story goes back billions of years.

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Given that I’m a living, breathing organism, I hold the deepest gratitude for some of the most important organisms on Earth. Without whom metazoan life would never have come into existence, your sweetheart included, and the lovely experience of love, across the spectrum, wouldn’t exist.

We live in a microbial driven world that only exists because Bacteria and Archaea tempered the previously hostile environment on early Earth to create atmospheric conditions that allowed metazoan (eukaryotic) life forms (humans included) to flourish and evolve. Bacterial and archaeal encoded enzymes catalyze all the major processes involved in global biogeochemical cycling, playing key roles in the carbon and nitrogen cycles, and producing approximately half of the oxygen in the Earth’s atmosphere.

The evolution of bacteria is tightly connected to their cognate bacteriophages, bacteria specific viruses. Bacteriophages or “phages” are the most abundant “organisms” in the biosphere and they are a ubiquitous feature of prokaryotic existence. Archaea are also infected by viruses, whether these should be referred to as ‘phages’ is debatable.

Ok, let’s begin our journey through the evolution of life on Earth starting with the incredibly amazing Cyanobacteria.

Cyanobacteria belong to the Cyanophyta phylum of bacteria. These blue-green algae obtain their energy through photosynthesis. The word “cyanobacteria” originates from the color of the bacteria (Greek: κυανός (kyanós) = blue). Cyanobacteria have an robust fossil record and have allowed us to understand the evolution of early life on Earth. In fact, the oldest known fossils are cyanobacteria from Archaean rocks of western Australia, dating back 3.5 billion years. You may find this surprising given that the oldest known rocks are only a slightly older, 3.8 billion years old!

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Fossil of a filamentous Palaeolyngbya

Just like other living organisms, bacteria can leave fossils and cyanobacteria have left a fossil record that extends far back into the Precambrian – the oldest cyanobacteria-like fossils known are nearly 3.5 billion years old, among the oldest fossils currently known. Cyanobacteria are larger than most bacteria, and may secrete a thick cell wall. More importantly, cyanobacteria may form large layered structures, called stromatolites (more or less dome-shaped) or oncolites (round). These structures form as a mat of cyanobacteria grows in an aquatic environment, trapping sediment and sometimes secreting calcium carbonate. When sectioned very thinly, fossil stromatolites may be found to contain exquisitely preserved fossil cyanobacteria and algae.

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Fossilized fungi spores

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Fossilized bacterial cells

Alright, so we know that ancient bacteria and archaea were able to temper the hostile environment of early Earth, generating the optimal atmospheric conditions for metazoan life, to evolve.

So how did complex eukaryotic life evolve to be so complex?

The answer lies in bioenergetics and endosymbiosis (engulfing of other cells for mutual benefit). Mitochondria are the prototypical bacterial endosymbionts that our cells have invited into our homes as permanent house guests. As payment for providing them food and shelter, Mitochondria produce most of the energy we use for survival. It should come as no surprise, that many animals living today also engulf algae for this purpose.

How does bioenergetics play into the evolution of expanded genome size and organismal complexity? Lane, N and Martin, W explain below (1):

“All complex life is composed of eukaryotic (nucleated) cells. The eukaryotic cell arose from prokaryotes just once in four billion years, and otherwise prokaryotes show no tendency to evolve greater complexity. Why not? Prokaryotic genome size is constrained by bioenergetics. The endosymbiosis that gave rise to mitochondria restructured the distribution of DNA in relation to bioenergetic membranes, permitting a remarkable 200,000-fold expansion in the number of genes expressed. This vast leap in genomic capacity was strictly dependent on mitochondrial power, and prerequisite to eukaryote complexity: the key innovation en route to multicellular life.” (1)

The enormous differences in the average genome size between prokaryotes and eukaryotes can be quantitatively highlighted in terms of the amount energy available per gene; the cost of expressing the gene. If you look at the “cost” of DNA replication on it’s own, it accounts for just 2% of a microbial cell’s energy budget during growth. Protein synthesis, in contrast, accounts for an astounding ~75% of a cell’s total energy budget (2).

The average bacterium, such as E. coli, has up to 13,000 ribosomes, whereas a human liver cell has 13 million on the rough endoplasmic reticulum alone. This large difference entails energetic costs that are orders of magnitude higher in eukaryotic cells.

As a linchpin of eukaryotic complexity is an immensely expanded repertoire of novel protein folds, protein interactions and regulatory cascades. Our “eukaryotic common ancestor increased its genetic repertoire by some 3,000 novel gene families. The invention of new protein folds in the eukaryotes was the most intense phase of gene invention since the origin of life. Eukaryotes invented five times as many protein folds as eubacteria, and ten times as many as archaea. Even median protein length is 30% greater in eukaryotes than in prokaryotes (1).”

“The transition to complex life on Earth was a unique event that hinged on a bioenergetic jump afforded by spatially combinatorial relations between two cells and two genomes (endosymbiosis), rather than natural selection acting on mutations accumulated gradually among physically isolated prokaryotic individuals (1). ”

I thank my life-giving bacteria, archaea, their respective cognate viruses and our permanent mitochondrial house guests everyday for allowing metazoan life on earth to evolve and exist.

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References:

1. Lane, N. and Martin, W. The energetics of genome complexity. Nature 467, 929–934 (21 October 2010) doi:10.1038/nature09486

2. Harold, F. M. The Vital Force: A Study of Bioenergetics (Freeman, 1986)

http://www.nature.com/…/jou…/v467/n7318/abs/nature09486.html

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Image:
A) Transmission electron micrograph of a eukaryote, a complex cell, the protist Euglena gracilis (scale bar, 5 µm). B) C) Fluorescence micrographs of DAPI-stained giant prokaryotes Epulopiscium fishelsoni (B) and Thiomargarita namibiensis. D) E) Transmission electron micrographs of mitochondria, site of chemiosmotic ATP synthesis in eukaryotes. All mitochondria retain core genomes of their own, which are necessary for the control of membrane potential across a circumscribed area of membrane, enabling a 104–105-fold increase in the total area of internalized bioenergetic membrane. D) A single folded mitochondrion in the dinoflagellate Oxyrrhis marina (osmium-fixed). E) Multiple mitochondria in the ciliate Paramecium bursaria. Photos: a, d, M. Farmer; b, E. Angert; c, H. Schulz-Vogt; e, R. Allen.

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