Sex jumpstarted Earth’s animal biodiversity
Popular Science...
Evolution is responsible for Earth’s stunningly diverse spectrum of life, but that wasn’t always the case. In fact, the earliest eras of living organisms were comparatively boring. The earliest known animals date back about 635 million years (during the Ediacaran Period), yet they look remarkably similar to their descendents 96 million years later at the dawn of the Cambrian.
Why did evolution remain so stable for so long? It might be simply because Earth’s first creatures simply weren’t having much sex.
“Life was pretty nice during the Ediacaran, so the need for sex was rather limited,” Emily Mitchell, a paleozoologist at the University of Cambridge, explained in a statement. “There was relatively little competition, so there was no real pressure to change anything.”
Along with her colleague Andrea Manica, Mitchell recently combined spatial analysis and laser scanning with machine learning to analyze 574-million-year-old fossils excavated from southernmost Newfoundland’s Mistaken Point. Their findings, published today in the journal Nature Ecology & Evolution, show that the earliest animals’ reliance on asexual reproduction kept things largely uniform, and reduced the struggle for resources.
They offered Fractofusus as a prime example. At over 6.5 feet tall, the fern-like creatures dwarfed most of their oceanic relatives and likely lacked organs or mouths. They also absorbed food from the surrounding water while remaining anchored in place, reproducing through clones distributed by stolons or runners like present-day strawberry plants.
“If you’re connected to your neighbor by these runners, then you’re sharing nutrients and you don’t need to compete with them,” said Manica.
From there, the team constructed a machine learning model to approximate how Fractofusus and its fellow Ediacaran animals possibly behaved through varying reproductive strategies. The program’s neural network then identified simulations that aligned with known fossil record diversity patterns. Known as Approximate Bayesian Computation let them basically travel back in time to estimate how animals proliferated and squared off for limited resources.
They now believe the Ediacaran Period’s overall tranquility (and sexlessness) began to get complicated as species gradually migrated from deep waters to shallower regions. Once there, ancient animals endured new stressors like temperature swings, nutrient deficits, tides, and even storms. Life then adapted to face these increased threats—and left behind more fossils. The story they tell indicates that environmental stress often precedes a rise in sexual reproduction versus other methods of procreation.
“When that happens, we can see a massive increase in dispersal distances as animals attempt to colonize new areas due to an increase in competition,” said Mitchell.
These shifting trends eventually ushered in what’s known as the Ediacaran “second wave” of animal evolution, which further amplified millions of years later during the Cambrian era, as animals started physically moving through their environments.
“If you’re suddenly in an environment where you’re essentially getting killed a couple of times per year, then that changes everything,” Mitchell explained.
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