Feather lice, in particular, seem to have split from their hitchhiking relatives sometime between and million years ago—right when little mammals were scurrying through the Cretaceous undergrowth and feathered dinosaurs were flocking around on land. Since feather lice evolved to feed on early birds and feather-covered, non-avian dinosaurs, they have had to change little to keep up with their hosts. A shell split into two halves, called valves, protects the invertebrate, but in the case of the brachiopod, these two halves are unequal in size.
Whether found in gravel, attached to kelp or clinging to the rock of a continental shelf, brachiopods are relatively rare today. There may be around different genera now living, but over 5, are known from a fossil record spanning million years. By about million years ago, brachiopods had become the dominant shelled animals in the seas—they were so thick in some places that their shells compose most of the sediment other fossils are found in—but that all changed with the worst mass extinction of all time.
Though the exact triggers are still debated, about million years ago a huge amount of greenhouse gases were dumped into the atmosphere, and the oceans became highly acidic. Brachiopods suffered, giving a foothold to the mollusk ancestors and cousins of modern clams and cockles.
Brachiopods have hung on in whatever crevices they could attach to but never managed to regain their dominance. Today these trees are represented only by one species, Ginkgo biloba , but this tree with fan-shaped leaves had its heyday when ferns, cycads and Jurassic dinosaurs dominated the landscape.
Modern Ginkgo trees are not very different from those that herbivorous dinosaurs may have fed on. A recent Paleobiology study by Wesleyan University paleobotanist Dana Royer and colleagues found that Ginkgo trees seem to do best in disturbed habitats alongside streams and levees, a habitat preference that may have been their downfall. Scientists know from living Ginkgo trees that they grow slowly, start reproducing late and are generally reproductive slowpokes when compared to more recently evolved lineages of plants that live in the same places.
Ginkgo trees may have simply been out-bred by other plants when suitable habitats opened up, but this makes it all the more remarkable that one species managed to survive to the present day. The duck-billed platypus truly looks as if it belongs to another era, if not another planet. In fact, when 19th-century European naturalists first saw stuffed specimens sent from Australia, some scholars thought the animals must be a joke.
Monotremes, like the platypus, are strange mammals. These archaic, egg-laying forms last shared a common ancestor with marsupial and placental mammals over million years ago, and rare fossils from Australia indicate that there have been platypus-like forms since million years ago. Though often reconstructed with a narrower-snout, the Late Cretaceous Steropodon was a close cousin of early platypuses.
A much closer relative to the modern platypus, known as Obdurodon , has been found in more recent rocks spanning about 25 to 5 million years ago. In this way, individual parts of a system evolve in unison to be both structurally and functionally compatible.
Eventually, over thousands and sometimes millions of years, the small improvements add up -- the simple, systematic process has produced an almost unfathomably complex organ. Recently, scientists have found clues to the evolutionary pasts of some of the most complex organs, helping to clarify how this process works. Learn More Life's Grand Design. Evolution does not stop once a species becomes a species.
Every population of living organisms is undergoing some sort of evolution, though the degree and speed of the process varies greatly from one group to another.
Populations that experience a major change in environmental conditions, whether that change comes in the form of a new predator or a new island to disperse to, evolve much more quickly than do populations in a more stable set of conditions. This is because evolution is driven by natural selection, and because when the environment changes, selective pressures change, favoring one portion of the population more heavily than it was favored before the change.
Learn how early humans evolved from Homo habilis, to Homo erectus, to Homo sapiens and developed basic survival tools. Learn how today's big cats evolved from prehistoric cat-like organisms, and what factors contributed to their evolution. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Photograph by James L. Amos Evolution is an important field of study for scientists.
Twitter Facebook Pinterest Google Classroom. Human Evolution Scientists who study early humans depend on fossil evidence to help them sort out how our ancestors evolved over time. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
Media If a media asset is downloadable, a download button appears in the corner of the media viewer. Text Text on this page is printable and can be used according to our Terms of Service. Interactives Any interactives on this page can only be played while you are visiting our website. Related Resources. View Collection. Theory of Evolution. View Article. Human Evolution. For instance, Hopkins points out that most species of opossum, like North America's Virginia opossum , are "dietary generalists.
I can eat trash. No trash? On the other hand, extremely specialized animals like giant pandas, whose diet is 99 percent bamboo , are more vulnerable to extinction if their preferred food is no longer available.
Weird Animal Question of the Week answers your questions every Saturday. If you have a question about the weird and wild animal world, tweet me , leave me a note or photo in the comments below, or find me on Facebook.
All rights reserved. Put That on My Bill You could say the platypus is a survivor: It's one of the few living descendants of an ancestor that diverged from all the other mammals about million years ago, Hopkins says. Share Tweet Email. Why it's so hard to treat pain in infants. This wild African cat has adapted to life in a big city. Animals Wild Cities This wild African cat has adapted to life in a big city Caracals have learned to hunt around the urban edges of Cape Town, though the predator faces many threats, such as getting hit by cars.
India bets its energy future on solar—in ways both small and big. Environment Planet Possible India bets its energy future on solar—in ways both small and big Grassroots efforts are bringing solar panels to rural villages without electricity, while massive solar arrays are being built across the country.
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