Moving in together - How the theory of endosymbiosis changed biology
The best partnerships in life happen when both parties benefit from the arrangement, working together as a team to become more than the sum of their parts. And as it is in relationships, so it is with life itself.
In fact, the diversity and success of life on this planet may be the result of cells buddying up and moving in together, combining their resources to create new organisms with advantageous new skills.
It’s an idea called endosymbiosis and it starts - as so many of our stories do - with the microscopists of the 19th century. Let’s take a closer look.
In 1883, German botanist Andreas Schimper published a volume of his observations of plants, focusing in particular on the colourful structures inside their cells such as the bright green, bean-shaped chloroplasts that are responsible for photosynthesis (presumably because they’re easy to see, what with being bright green…).
In a footnote, he mentions a certain Professor Schmitz telling him that chloroplasts in algae appear to be produced by existing structures dividing in two and being separated into new cells, a bit like bacteria, rather than being built from scratch every time.
As Schimper went on to observe, the same thing seems to happens in more complex plants too. He wrote, “If this is definitely proven... then this would be a symbiosis” - a Greek word meaning ‘living together’, by which he meant that chloroplasts were more like distinct little bacterial cells living within their larger plant cell hosts.
Building on this idea, in 1905 Russian botanist Konstantin Mereschkowski first published his theory explaining that chloroplasts may have arisen as a result of symbiosis, with a smaller photosynthetic bacteria being engulfed by a larger cell at some point back in the mists of time.
A decade or so later, biologists were training their sights on another structure inside complex cells: mitochondria. Somewhat similar in shape and structure to chloroplasts, mitochondria are the power stations of cells, effectively burning sugar and oxygen to generate the energy that fuels all the processes of life.
Noting the similarities with chloroplasts and little bacteria, Frenchman Paul Jules Portier and American Ivan Wallin put forward the idea that mitochondria too were the result of an ancient engulfment of one cell by another.
But it’s not enough to just look like bacteria and appear to reproduce like them. The next step came in 1959, when Ralph Stocking and Ernest Gifford discovered that chloroplasts contain their own DNA - by that point known to be the stuff that genes are made of, and previously only thought to live in the cell nucleus.
The discovery of DNA in chloroplasts and mitochondria was the first piece of hard evidence that these subcellular structures might have had a previous existence as free-living cells in their own right.
But if they were now inseparably tangled up inside larger cells, how had they got there? And what did this cellular partnership mean for our understanding of the origins of complex life?
Thanks to the burgeoning molecular biology revolution of the 1950s and 60s, most biologists became obsessed with the biochemical side of life, breaking open cells to study the DNA and proteins inside them.
But one curious scientist shunned this purely gene-based perspective and took a broader look at life. Her name was Lynn Sagan - at the time the wife of astronomer and science writer Carl Sagan, and later known as Lynn Margulis after her second marriage.
She drew together evidence from a wide range of sources - microscopic, molecular and from the fossil record - to suggest that there was a heck of a lot more symbiosis going on than anyone had previously imagined. Not only that, but symbiosis had played a radically important role in the evolutionary history of life on earth.
Engulfing photosynthesising bacteria to create chloroplasts enabled early plants to produce the boost in oxygen that sparked major evolutionary transformations for the animals that consumed it thanks to their own endosymbionts, mitochondria..
Margulis’ theory did not go down well. Her paper outlining her ideas was rejected more than a dozen times before finally being published in 1967.
Her view that symbiosis was perhaps the most powerful driving force in evolution, with molecular mergers and acquisitions resulting in the emergence of new and more complex beings. Unfortunately, very few others agreed.
Margulis’ ideas about evolution proceeding by co-operating and cellular buddying-up bringing about rapid biological innovation ran counter to the prevailing view of many evolutionary biologists in the 1960s and 70s, and they fought back hard.
The so-called neo-Darwinists like Richard Dawkins and John Maynard Smith held that evolution created new species through the slow, gradual tick of genetic change and competitive selection - survival of the fittest rather than such bold collaborative leaps.
But Margulis didn’t accept that this slow, selfish creep was enough to bring about the big evolutionary changes that must have taken place as new species emerged.
In her own words, "Life did not take over the globe by combat, but by networking".
Margulis’ combative personality and contrarian attitude didn’t exactly make her popular with the scientific establishment (perhaps combined with the fact she was a woman in the predominantly male world of academic research).
But as more and more hard evidence came to light through the 1980s and beyond, such as the discovery that the genes in mitochondria and chloroplasts are more closely related to the DNA in bacterial rather than complex cells, Margulis’ outsider theory of endosymbiosis came to be accepted as the established narrative.
There are still arguments around the edges - such as exactly when and how many of these fusions have happened over the history or life on earth, whether whip-like structures called flagella are the remnants of ancient engulfed spiral-shaped bacteria, and whether the cell nucleus itself is an ancestral engulfed virus - but the basics are thought to hold up.
By the time Margulis died of a stroke in 2011 at the age of 73, endosymbiosis was broadly accepted as the explanation for the origins of organelles like mitochondria and chloroplasts in eukaryotic cells.
And many species are now known to be made up of cells locked together in symbiotic relationships, including algae, lichen, unicellular organisms and more.
Margulis also made another important contribution to science, working with environmentalist James Lovelock to develop the so-called Gaia hypothesis - the idea that the Earth itself and all its inhabitants form a synergistic and self-regulating, complex system.
But although her once-controversial contributions to biology have become part of the scientific canon, in her later years Margulis became notorious for making controversial statements without solid evidence, such as promoting 9/11 conspiracy theories.
While that stuff is probably best forgotten, her vivacity and tenacity for a broader view of biology should always be remembered. She became widely regarded as a leading figure in biology, and received a National Science Award from U.S. President Bill Clinton in 1999.
Her obituary in the journal Nature describes her as an “independent, gifted and spirited biologist who learned as early as the fourth grade to “tell bullshit from ... real authentic experience”... With courage, intellect, a twinkle in her eyes and considerable fortitude, she changed our view of cellular evolution.”
Even Richard Dawkins recognised her persistence in sticking to her scientific guns, saying “I greatly admire Lynn Margulis' sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy… This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it.”
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