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Life on Earth Part 2


 
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Professor Philip Hugenholtz, Director of the Australian Centre for Ecogenomics at the University of Queensland, places the tree of life under the microscope and uncovers some major facts about microorganisms.    
The first life on Earth was single celled, thought to be forerunner of the domains of bacteria, archaea and eucarya. Our first physical records are of microbial mats which are evidence of huge communities of microbes aggregated together for common good (eg survival by not being buried by layers of sediment). Geological records include the Dresser Formation in the Pilbara WA with fossilised examples of mat structures dating back over 3.5 billion years, and the famous stromatolites in Shark Bay WA. Colonies of microorganisms gradually built up these mound shaped objects that have survived millions of years - quite an achievement for what are basically biofilms produced by single celled microscopic organisms.

 

Stromatolites at Shark Bay 
These rock-like mounds have been slowly built up by huge colonies of bacteria over time and represent one form of microbial mats.
The stromatolites at Shark Bay are 'recent' being only 2,000 to 3,000 years old. They represent the world's most outstanding examples of living marine stromatolites and are similar to life forms found on Earth up to 3.5 billion years ago. Termed 'living fossils' they provide insights into the history of life on Earth. Their presence in Shark Bay were important in declaration of the region being declared a World Heritage Area.
(basic text and images from: http://www.sharkbay.org.au/nature-of-shark-bay-stromatolites.aspx


 
Below: Slice through microbial mat from Guerrero Negro hypersaline evaporation pond.    
While we lack physical fossil records of the evolution of both diverse singled celled and later multicellular organisms, the advances in knowledge of and technology in genetic research have made it possible for scientists to look for genes common to all different forms of life, thereby building up making a tree of life to show this evolutionary history. Humans, for instance, are distantly related to e coli and e coli is more closely related to salmonella. 

 

Microbial mats served similar purposes in a different format. A famous example is at Guerrero Negro, Mexicom which stretches for hundreds of kilometres in the ocean off Mexico and is 20cm thick.
Images of the hypersaline evaporation pond and mat slice mfrom The Pace Lab University of Colorado, for which the Bacterial, Archaeal, and Eukaryotic microbial diversity of the hypersaline microbial mat of Baja California, Mexico is a long standing research focus.


 
The trees can be traced back to find the last common universal ancestor, now thought to be a pre-cellular macromolecule which evolved into more complex associations of macromolecules (proteins, DNAs, polymers) that eventually became catalytic as theorised by Carl Woese. Such macromolecules developed into single cells by putting an envelope on the outside. 

 
Microorganisms are hardier than people think, mould or scum being a case in point. Different species not only survive but live in temperatures from sub-freezing to above boiling point and can survive without any oxygen or in super-oxygenated environments. They can even survive in radioactive bioreactors. Microorganisms (unwittingly no doubt) work as a team, sharing nutrients and taking up different functional parts of the food web. There are many advantages in living together as a community, protection being one. A good example of their success is the microorganisms in the human GIT, a winning strategy for them from an evolutionary point of view as they work together and simultaneously ensure human life. The moment a microorganism forms a stable relationship with another it has access to all of its genes and there is often wholesale recruitment of genes from the engulfed organism which are then taken into the main genome of the host organism. If that organism survives it had an advantage so in modern times we are looking at the winners of these experiments over the ages, something the huge populations of microorganisms could afford to do.

In terms of survival, climate change is a case in point. The Earth has seen many big changes, there always being winners and losers, organisms that can’t adapt dying out. In this regard microorganisms are in a much safer position under climate change than humans who are less adaptive in terms of physiology. Many microorganisms will, however, happily adapt to such changing circumstances.

 

 
Above Left: 3.48 billion-year-old macrostructures from the Dresser Formation in Pilbara, Western Australia. Right: possible modern equivalents. Scale bars – 1 cm. Image credit: Nora Noffke et al.Bibliographic information: Nora Noffke et al. Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia. Astrobiology, published online November 8, 2013; doi: 10.1089/ast.2013.1030 and repoorted in Sci News, November 12, 2013 http://www.sci-news.com/paleontology/science-fossilized-ecosystems-australia-01537.html

 
Reproduction is a key feature of life. Viruses, which are more or less inert by themselves, need hosts in order to reproduce. Viruses therefore, could not have evolved before those hosts were available. This is an extreme adaptation, the viruses becoming more and more parasitic, eventually throwing away most of their genetic machinery and relying on hosts to do it for them.
Indeed, viruses could be thought of as the ultimate parasite!

New forms of life (microorganisms) are being discovered through molecular methods. Mitochondria, for instance, are not an independent organism. They are fully integrated into human cells and cannot live as a free entity (which is how they started out). Aphids have a type of bacteria that is also symbiotic, completely dependent on the aphids for life, relying on their hosts to make things like amino acids. Such bacteria are halfway between an independent living organism and a virus, which is completely dependent on its host.

Viruses have undergone such extreme changes that their genes aren’t recognisable making them hard to place on the tree of life. Viruses do all the changing of their DNA while they are reproducing within the hosts. Once they are mature virions they have no way of changing their DNA. They have the ability to use host DNA and the DNA of other viruses to recombine into new DNA and because they are around in huge numbers they can afford to play around with this DNA reconfiguration from an evolutionary point of view. That is why flu shots keep changing. The virus changes to avoid the mechanisms the host is using to repel them and the virus can change much more rapidly its host. Microorganisms are a key feature of the tree of life.

Professor Philip Hugenholtz was interviewed for A Question of Balance by Ruby Vincent. Images from sources stated in the text. Summary text by Victor Barry, April 2016.

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