Graced with hardy exoskeletons, trilobites preserve some of the oldest evidence of evolutionary innovations common today: heads, mouths, gills, legs and above all eyes. Described in 2017, a three-dimensional trilobite eye fossil preserved enough internal detail to show a work-in-progress compound eye belonging to . Though it had the same general structure as modern bee and dragonfly eyes, the ancient peeper lacked evidence of the tightly packed lenses needed for image formation. Instead, the half-a-billion-year-old arthropod probably used its eyes for little more than movement or obstacle detection. This species was collected from near the base of . A slightly younger (geologically speaking) trilobite specimen showed eye structure closer to that of modern dragonflies.
Two bug species probably made their way to the Hawaiian Islands around the turn of the last century: (a parasitic fly from North America) and (a cricket from Oceania). Like boys everywhere, the male crickets tried to win girlfriends with noise, in their case by rubbing their wings together. But in the presence of the flies, who find the sound as attractive as female crickets do, and whose larvae like to burrow into the males and kill them from the inside, silence turned absolutely golden. Within as few as 20 generations, male crickets on two islands got much quieter, actually losing their chirping abilities. About 50 percent of the males on Oahu, and about 95 percent of the males on Kauai lost the wing structures that make the love songs humans hear as chirps. Not only did entomologists observe these changes over a short time, thanks to the bugs' short lifespans, but they could also see the differences in the wings between the island populations. A study published in 2014 reported that DNA analysis showed different markers in the genomes of the Oahu and Kauai crickets — an example of convergent evolution, where different populations independently evolve similar traits.
Soft inheritance: challenging the modern synthesis - …
Denisovans are an amazing discovery. Neanderthals have long fascinated anthropologists and the general public alike. Studies uncovering ancient trysts between those populations and our direct ancestors will no doubt continue. But it's important to view Denisovans and Neanderthals in their proper perspective. In the autumn of 2013, Adam Van Arsdale, anthropology professor at Wellesley College and longtime researcher at Dmanisi Cave, explained to students of Wellesley's massive open online course (MOOC) on human evolution:
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Ongoing DNA studies present an increasingly complicated picture of human history. In rapid succession, two 2013 studies found that (1) Denisovans bred with Neanderthals, ancestors of people now living in East Asia and Oceania, and another group of extinct archaic humans who were genetically dissimilar to both Neanderthals and modern humans, and (2) Mitochondrial DNA (inherited through females) from Spain's Sima de los Huesos Cave showed a link to Denisovans, a link reaffirmed in a 2016 study. Ancient Eurasian hominin DNA also showed evidence of inbreeding, perhaps a common event when local populations were isolated by a tough environment. In short, early humans were a randy bunch. Given the varying levels of Denisovan DNA in modern populations, Pääbo suggests that early migrants out of Africa might have met Denisovans while traveling along the coast of southern Asia.
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In 2010, the Natural History Museum at the Smithsonian opened its state-of-the-art, awesome hall of human origins. Within weeks, a new study led by Pääbo described a previously unknown hominin species from Denisova Cave in Siberia. Dubbed Denisovans, these enigmatic humans were identified from DNA analysis of fossils too fragmentary to give any idea of what the hominins looked like, but they apparently lived at the same time as Neanderthals and the direct ancestors of modern humans. Considering Denisovan fossils turned up in Russia, you might expect Denisovan DNA to occur in large parts of Russia and China, but it has proven relatively scarce in modern human populations outside of Southeast Asia.
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Neanderthals and the ancestors of modern humans may have been "at the edge of biological compatibility," and their hybrid progeny may have eventually become infertile, according to one group of researchers. And yet DNA analysis of a roughly 40,000-year-old modern human jawbone from Pestera cu Oase Cave in Romania showed strong evidence of interbreeding. If you make a baby, you contribute large stretches of your DNA to your child. But over generations, those chunks of DNA you have contributed will get broken into smaller and smaller pieces. The jawbone showed long, uninterrupted stretches of Neanderthal DNA, prompting anthropologist Erik Trinkaus to argue in 2015 that the mandible apparently belonged to the great-great-great-grandson of a Neanderthal.