The day before the NASA team announced they found 7 planets around the Trappist-1 red dwarf star (three in the habitable zone)1, I went to the Duquesne University Darwin Day lecture by Dr. Nick Lane. If you want, you can watch Dr. Lane’s lecture online.
There are an amazing number of things I want to talk about from that lecture, but with the NASA announcement, I’d like to bring up one: Dr. Lane feels that bacterial-style life may be common, but multicellular life is exceedingly rare.
A quick summary goes like this: Life started on Earth 4.1-3.8 billion years ago. The big argument is whether life bothered to wait for the Late Heavy Bombardment (LHB) to end. The constant bombardment of space rubble doesn’t sound like the best time to start life, but there’s good evidence2 it either started before the LHB end or incredibly shortly thereafter.
A lot of interesting biochemistry went on in cells for the next 2.3 billion years, but it was all single-cell, with maybe a few mats or poorly separated colonies of single cells cooperating–but hardly differentiating into specialized tasks.
Then, something different happened. Multicellular life went hog-wild and became a dominant force. What happened?
According to the endosymbiosis theory of Dr. Dr. Lynn Margulis in 1967, an archaeal cell somehow grabbed a bacteria and didn’t destroy it. The two became symbiotic, and the bacteria became the powerhouse for the new cell–a primitive eukaryote. With this division of labor, the new cell took off, evolved, and became multicellular life.3
It took 2.3 billion years. It only happened once. Dr. Lane thinks that, while the odds are good we will find life somewhere–Mars or Europa or Titan4, or Trappist-1e–it will probably be bacterial or algal.
The step to mitochondria was too hard. It might be “The Great Barrier” that explains why the universe isn’t over-run by aliens. And once the step occurred, it might have been a long, difficult fight to survive it. Mitochondria lost most of their DNA to the cell nucleus, but not all. Reaching that balance must have been incredibly difficult. And mitochondria are dangerous. They’re very tiny, but the energy they produce is tremendous on a cellular scale. They keep those few genes to themselves to keep something “bad” from happening. When cells are programmed to die, the mitochondria are chief players in the destruction. And mitochondria leak out their free radicals, damaging the cell and causing aging. Mitochondria are playing with fire–or at least oxidation!
Dr. Lane may be right. He knows far more than I do. But I’d argue it might not be that bad.
First, life had to invent a lot of biochemistry. Most of the interesting biochemistry in humans and other eukaryotes can be found in yeast. The “last common ancestor” that grabbed a bacteria to become a eukaryote was extremely sophisticated biochemically. There may have been many steps necessary before a cell could even successfully grab the free-living precursor of mitochondria.
Second, a lot of bad things happened during that time. The creation of photosynthesis sucked the carbon dioxide out of the air, causing the Earth to freeze over. There’s a lot of debate as to how many times life was nearly killed off and earth became “Snowball Earth,” but it was at least once and probably a lot more. Any early eukaryotic-like cells with symbiont bacterial energy sources may have died off during those Snowball Earth periods. Survival may have favored the less energetic bacteria and archaeons.
Third, once the primitive eukaryotes got past the “let’s learn to cooperate with this thing I just swallowed” stage, they may have been very difficult to compete against. Mitochondria provide great efficiency and power. Bacteria can still survive as bacteria–and by preying on eukaryotes, but something trying to bridge the gap between them might be in a very precarious position, especially during the time when it’s learning to deal with the engulfed bacteria.
Fourth, it did happen more than once. Some bacterial cell grabbed a cell of algae and, in a process remarkably similar to what happened with the mitochondria, turned it into a chloroplast. So there were two cell lines–those with mitochondria and those with chloroplasts. Yet today, we see plants with chloroplasts and mitochondria. It turns out that mitochondria-containing cells and chloroplast-containing cells merged several times. This process is similar to the one that created chloroplasts and mitochondria. And finally, someone found a bacteria that has engulfed another bacteria. It’s a crude symbiosis, and may have happened “recently,” where “recent” is defined in something between biologically “recent” and geologically “recent.”
Dr. Lane is probably right. Multicellular life is difficult.
But it may not be hopeless.
If Dr. Lane is right, then the “Great Barrier” is behind us. If I’m right, then perhaps there are multiple Great Barriers instead of just one. Examples of these barriers might be, developing intelligence, developing technology, surviving technology, and becoming starfaring.
With the Doomsday Clock two and a half minutes to midnight, it might be more comforting to think the Great Barrier is behind us and statistically, we will survive the coming years.
1.5 billion years ago: Mitochondria
Dr. Lynn Margulis
I'm not entering a 12-Step program for science denial, but I feel that way. I'm a "facts" person. My natural inclination to...