How Ancient Life May Have Come About
David Huntley | Shutterstock
The so-called last universal common ancestor (or LUCA), may be just a fantasy, if life emerged from a collective state.
April 19, 2012 --
Forty years ago this week, the crew of Apollo 16 captured this image of Earth rising above the lunar landscape. The Apollo missions enabled humanity to see for the first time our planet as it appears from space. As Apollo 13 commander Jim Lovell once said: “When I was orbiting the moon and could put my thumb up to the window and completely cover the Earth, I felt a real sense of my own insignificance. Everything I'd ever known could be hidden behind my thumb.” As we approach Earth Day on April 22, we look at the efforts of people throughout the ages to explore, understand and portray our world and its place in the Universe.
Trustees of the British Museum (image rotated
Babylonia Believed to be the earliest known representation of Earth, this stone tablet from Babylon shows the world as a disc, surrounded by a ring of water called the "Bitter River." The world is dominated by the area surrounding Babylon itself, and the Euphrates River bisects most of the inner circle. Unearthed in southern Iraq in the late 1800s, the tablet is housed in the British Museum.
Sixteenth-century interpretation of Ptolemy's
Celestial Spheres In his 2nd century treatise, the "Almagest," Claudius Ptolemy proposed an explanation for the apparent movement of stars and planets, in which Earth was central and immovable, and surrounded by, at progressively greater distances, the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn and a sphere of ‘fixed stars.’ This geocentric view of the cosmos did not meet its first real challenge until Copernicus proposed that the planets revolved around the Sun, and Galileo used his telescope to observe the phases of Venus.
Library of Congress, via the History Blog
Flat Earth The Greek philosopher Aristotle determined that Earth was spherical and not flat almost 2,500 years ago. The notion of a flat earth retained at least a few die-hard devotees for a surprisingly long time. For example, this 1893 map by Orlando Ferguson, recently acquired by the Library of Congress, cites “Scripture that condemns the globe theory” and promotes a book that “knocks the globe theory clean out.”
ANALYSIS: What if Earth Were a Cube?
De Costa, B.F. (September 1879). "The Lenox G
Lenox Globe It is popularly believed that ancient cartographers filled in unknown and unexplored areas of the world with the phrase ‘Here be dragons’. In fact, only one known ancient map – the so-called Lenox Globe, which is believed to date to around 1510 - displays the phrase ‘HC SVNT DRACONES’, from the Latin “hic sunt dracones.” (The phrase is written near the equator on the eastern cost of Asia.) Some nineteenth-century writers, however, believed that it referred, not to dragons, but to the ‘Dagroians’, a people who “feasted upon the dead and picked their bones.”
PHOTOS: Sea Monsters Real & Imagined
Image Database of the Kano Collection, Tohoku
Terra Australis Incognita In this copy of a 1602 map that was created on behalf of China’s Wanli emperor by Italian Matteo Ricci and collaborators, the familiar outlines of most of the world’s continents are coming into shape, although obviously many details remain unfinished. To the map’s makers, however, the likes of Australia, New Zealand and Antarctica are not even figments of the imagination, replaced instead by an enormous southern landmass. The notion of an unknown southern land – a terra australis incognita - was first mooted by Aristotle in 322 BCE; not until 1820 did Fabian von Bellingshausen become the first man to see the Antarctic continent.
South Pole For centuries, gaps in maps were filled by explorers who set out across land and sea, often at immense personal risk. The true nature of “Terra Australis” had long been established by the time Robert Falcon Scott and comrades stood at the South Pole on Jan. 17, 1912; but existing knowledge could not diminish the terrible toll the conditions exacted on the men. “Great God!” wrote Scott in his journal, “this is an awful place.” All five members of Scott’s polar team died before they could reach their base camp.
PHOTOS: Forgotten Discoveries of Scott's Antarctica
Moscow at night Time and technology have enabled us to explore, not just across the surface of the globe or even beneath its waves, but from on high. Here, Moscow is seen at night from the International Space Station, flying at an altitude of approximately 240 miles on March 28, 2012. A solar array panel for the space station is on the left side of the frame. The Aurora Borealis, airglow and daybreak frame the horizon.
Pale Blue Dot In contrast to earlier suppositions about our place in the firmaments, we know now that our globe is not at the center of the cosmos, and that other celestial bodies are not attached to interlaced spheres that rotate around us. We are but one world among many, in one solar system among many, in one galaxy among many. In this image, taken by the Voyager I spacecraft from a distance of 4 billion miles, Earth is but a speck – a pale blue dot – in the cosmic night.
NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring
Blue Marble If satellite images of Earth now seem almost routine, they never lose their ability to enthrall. This picture of the western hemisphere was captured on January 25 by NASA’s latest Earth observation satellite, Suomi NPP. By February 1, it had registered over 3 million views on Flickr – testament to the beauty and fascination of our Blue Marble.
PHOTOS: Earth's Blue Marble Beauty
A family tree unites a diverse group of individuals that all carry genetic vestiges from a single common ancestor at the base of the tree. But this organizational structure falls apart if genetic information is a communal resource as opposed to a family possession.
Some evidence suggests that early evolution may have been based on a collective sharing of genes. A group of researchers are now searching for clear genetic vestiges from this communal ancestry.
But it's hard to shake our fascination with family trees.
My father used to travel for work, and when he arrived in a new city, he'd open up the phone book and check for anyone listed with our uncommon last name. Occasionally he'd get a hit and brazenly call them up to ask: "Are we related?"
The answer was always yes, with the common link often being my great grandfather.
Like my father, biologists are curious about family ties, but they go about it in a more systematic way. Rather than phone books, they sift through genetic codes from humans to bacteria and a lot in between. The main question is: Are the commonly held genes similar enough to point to a common origin?
The answer has always been yes. The implication is that we all belong to some universal tree of life. And at the base of this tree — some have imagined — there sits a mild-mannered microbe that lived more than 3 billion years ago, unaware that its genes would be the starting point of an entire planet's worth of highly differentiated life.
However, this organism, the so-called last universal common ancestor (or LUCA), may be just a fantasy.
"Our perspective is that life emerged from a collective state, and so it is not at all obvious that there is one single organism which was ancestral," said Nigel Goldenfeld from the University of Illinois at Urbana-Champaign.
The organisms belonging to this collective state would have shared genetic information from neighbor to neighbor, rather than solely from parent to offspring. Goldenfeld is leading a new NASA Astrobiology Institute (NAI) team that aims to provide a clearer understanding of this early stage of evolution.
"We are hoping to find fossils of the collective state in the genomes of organisms," Goldenfeld said.
Goldenfeld's team will be performing genetic studies that will try to tease out signatures of community-based evolution. They will complement this field and laboratory work with theoretical modeling and computer simulations.
"The ultimate goal is to understand how our planet's biochemistry is an instantiation of the universal laws of life, thus addressing the question of whether life is an inevitable and thus widespread outcome of the laws of physics," Goldenfeld said.
A time before Darwinism
It might sound strange that an organism's genetic code could be the result of "crowdsourcing." We are more familiar with traditional reproduction, as practiced by the birds and the bees. (Code of Life: 10 Animal Genomes Deciphered)
In so-called "vertical gene transfer," an organism inherits its genome from its parents, but it does not receive an exact copy. Small changes enter the code through reproductive mixing and mutations. This "descent with modification," as Darwin put it, eventually allows a population of interbreeding organisms (or species) to evolve.
If every snippet of DNA was solely the product of descent with modification, then every organism could be placed on a tree of life stemming from a single ancestor. But as it turns out, "different genes go back to different ancestors," said Peter Gogarten of the University of Connecticut, who has done extensive work on comparative genetics.
How is that possible? It can happen if organisms share genes. Imagine a gene belonging to members of a specific family tree. One day, this gene becomes isolated and gets picked up by another organism with a different family tree. No reproduction between partners takes place — only an "adoption" of a specific gene.
This so-called "horizontal gene transfer" is quite common among bacteria and archaea, as exemplified by antibiotic resistance. When a specific bacterium develops a defense against some drug, the corresponding gene can pass horizontally to others in the same colony.
A 2008 study in the journal Proceedings of the National Academy of Sciences (PNAS) found that 80 percent of the genes in bacteria were horizontally transferred at some point in the past.
Complex organisms also exhibit evidence of horizontal (or lateral) gene transfer, albeit to a lesser extent. Researchers have shown that ancient ancestors of plants and animals "swallowed up" other bacteria to form symbiotic relationships, which eventually resulted in specialized cellular components, such as mitochondria and chloroplasts.
In his work, Gogarten has shown that horizontal gene transfer turns the tree of life into a thick bush of branches that interweave with each other. Many of these branches terminated long ago due to extinction, but some of their genes live on in us, thanks to horizontal gene transfer.
Several studies suggest that horizontal gene transfer was more prevalent in the past when nothing but single-celled organisms inhabited the Earth.
"I like to think of early life as being more like an undifferentiated slime mold," Goldenfeld said. "Such a communal form of life would have no meaningful family tree, because it is the community that varies in descent, not individual organismal lineages." (7 Surprising Theories on the Origin of Life)
The so-called last universal common ancestor (or LUCA), may be just a fantasy, if life emerged from a collective state.David Huntley | Shutterstock
The late Carl Woese, a colleague of Goldenfeld, was one of the first scientists to propose that early life leaned heavily on horizontal gene transfer. Woese passed away in December of last year. He is perhaps best-remembered for classifying life into the now-well-accepted domains of bacteria, eukaryotes (plants, animals, fungi and protists) and archaea.
In 1987, Woese wrote about the consequences of rampant horizontal gene transfer. In such a scenario, "a bacterium would not actually have a history in its own right: It would be an evolutionary chimera."
A "chimera" is the name of a creature from Greek mythology that mixed together features of a lion, a goat and a snake. This hybridization presumably gave the chimera an advantage over its "competitors."
In a 2006 PNAS paper, Kalin Vetsigian, Woese and Goldenfeld showed that microbial chimeras may also have an advantage over their biological counterparts. The researchers used computer models to demonstrate that the genetic code could evolve more efficiently if organisms shared their genes collectively. Horizontal gene transfer turned out to be a better "innovation-sharing protocol" than vertical (Darwinian) transfer.
Now, with his NAI team, Goldenfeld wants to confirm these simulations with genetic studies. Specifically, they will target archaea, whose genes have yet to be scrutinized as closely as those from the other domains, Goldenfeld said.
The group is particularly interested in the question of how the ability to evolve originally developed. The "evolution of evolution" sounds like a chicken-and-egg problem — especially if you think, as Goldenfeld does, that life is by definition something capable of evolving.
However, evolution can utilize different mechanisms to achieve the same goal. Goldenfeld's team will try to recover some of life's former evolutionary phases by stressing cells and then seeing how their genomes rearrange in response.
However, DNA evidence is just one aspect of this five-year research project.
"We want to understand how evolution works before there were species or maybe even genes," Goldenfeld said. "So this is going beyond 'origin of species' approaches to evolution, such as population genetics."
How does one study evolution without genetics? One considers the "rules of the game" that the genetic code is just one manifestation of. Goldenfeld calls this "universal biology." It is an attempt to distill from our specific biochemistry the general physical laws that animate matter.
Being a physicist, Goldenfeld gives the example of thermodynamics. Life must obey conservation of energy and the law of increasing entropy, which will certainly influence how organisms optimize their use of resources.
Other rules involve how to control the amount of variation in the genome from one generation to the next. Too little variation, and organisms can't adapt to changes in the environment. Too much variation, and organisms can't retain useful traits.
The team can place different sets of rules into a computer simulation and see what sort of artificial life appears. Goldenfeld believes that formulating the principles of universal biology may help answer one of the biggest questions of all.
"We would like to have a better understanding of why life exists at all." Goldenfeld said. "Is it a phenomenon that should be generic, like the formation of a crystalline solid, or is it something rare and bizarre?"
This is of special interest to astrobiologists, who wonder about the likelihood that we are not alone. If life is eventually found elsewhere, Goldenfeld thinks we'll have a few things in common. (Mars Discovery Raises Question: What Is Life?)
"The principles of universal biology should be applicable to all life irrespective of whether it is carbon chemistry-based or something stranger," he said.
Something stranger? Okay, so maybe that means they won't be in the phone book.
This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program. More from LiveScience:
Extreme Life on Earth: 8 Bizarre Creatures
The Surprising Complexity of LUCA
UFO Quiz: What's Really Out There
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