Little is known about how this happened. However, small worms called Acoela may be the closest surviving relatives of the first ever bilateral animal. It seems likely that the first bilateral animal was a kind of worm. Vernanimalcula guizhouena , which dates from around million years ago, may be the earliest bilateral animal found in the fossil record. The Bilateria , those animals with bilateral symmetry, undergo a profound evolutionary split.
They divide into the protostomes and deuterostomes. The deuterostomes eventually include all the vertebrates, plus an outlier group called the Ambulacraria. The protostomes become all the arthropods insects, spiders, crabs, shrimp and so forth , various types of worm, and the microscopic rotifers. The first hole that the embryo acquires, the blastopore, forms the anus in deuterostomes, but in protostomes it forms the mouth.
The earliest known fossils of cnidarians , the group that includes jellyfish, sea anemones and corals, date to around this time — though the fossil evidence has been disputed. Strange life forms known as the Ediacarans appear around this time and persist for about 33 million years. A small group breaks away from the main group of deuterostomes, known as the Ambulacraria.
This group eventually becomes the echinoderms starfish, brittle stars and their relatives and two worm-like families called the hemichordates and Xenoturbellida.
Fossilised animal trails suggest that some animals are moving under their own power. As the first chordates — animals that have a backbone, or at least a primitive version of it — emerge among the deuterostomes, a surprising cousin branches off. The sea squirts tunicates begin their history as tadpole-like chordates, but metamorphose partway through their lives into bottom-dwelling filter feeders that look rather like a bag of seawater anchored to a rock.
Their larvae still look like tadpoles today, revealing their close relationship to backboned animals. The Cambrian explosion begins, with many new body layouts appearing on the scene — though the seeming rapidity of the appearance of new life forms may simply be an illusion caused by a lack of older fossils.
The first true vertebrate — an animal with a backbone — appears. It probably evolves from a jawless fish that has a notochord, a stiff rod of cartilage, instead of a true backbone.
The first vertebrate is probably quite like a lamprey, hagfish or lancelet. Around the same time, the first clear fossils of trilobites appear. These invertebrates, which look like oversized woodlice and grow to 70 centimetres in length , proliferate in the oceans for the next million years. They probably look like eels. Fossil evidence shows that animals were exploring the land at this time. The first animals to do so were probably euthycarcinoids — thought to be the missing link between insects and crustaceans.
Nectocaris pteryx , thought to be the oldest known ancestor of the cephalopods — the group that includes squid — lives around this time.
The Great Ordovician Biodiversification Event begins, leading to a great increase in diversity. Within each of the major groups of animals and plants, many new varieties appear. Plants begin colonising the land. Fish split into two major groups: the bony fish and cartilaginous fish. The cartilaginous fish, as the name implies, have skeletons made of cartilage rather than the harder bone.
They eventually include all the sharks, skates and rays. The bony fish split into their two major groups: the lobe-finned fish with bones in their fleshy fins, and the ray-finned fish. The lobe-finned fish eventually give rise to amphibians, reptiles, birds and mammals. The ray-finned fish thrive, and give rise to most fish species living today. The common ancestor of lobe-finned and ray-finned fish probably has simple sacs that function as primitive lungs, allowing it to gulp air when oxygen levels in the water fall too low.
In ray-finned fish, these sacs evolve into the swim bladder, which is used for controlling buoyancy. Active feeding by well-armored animals like trilobites may have further disrupted the sea floor that the soft Ediacaran creatures had lived on. Unique feeding styles partitioned the environment, making room for more diversification of life. While Waptia scoured the ocean bottom, priapulid worms burrowed into the sediment, Wiwaxia attached to sponges, and Anomalocaris cruised above.
Many of these odd-looking organisms were evolutionary experiments, such as the 5-eyed Opabinia. However, some groups, such as the trilobites, thrived and dominated Earth for hundreds of millions of years but eventually went extinct.
Stromatolite reef-building bacteria also declined, and reefs made by organisms called brachiopods arose as conditions on Earth continued to change. However, despite all the changes that were to come, by the end of the Cambrian nearly all existing animal types, or phyla, mollusks, arthropods, annelids, etc.
Skip to main content. Smithsonian Institution. Early Life on Earth — Animal Origins. An Oxygen Atmosphere When cyanobacteria evolved at least 2. Multicellular Life However, other innovations were occurring. The First Animals These clusters of specialized, cooperating cells eventually became the first animals , which DNA evidence suggests evolved around million years ago.
Ediacaran Biota By about million years ago the Ediacaran Period there was a proliferation of other organisms, in addition to sponges. The End-Ediacaran Extinction However, about million years ago, most of the Ediacaran creatures disappeared, signaling a major environmental change that Douglas Erwin and other scientists are still working to understand.
The Cambrian Explosion The Cambrian Period million years ago witnessed a wild explosion of new life forms. Related Resources. Video: The Cambrian Explosion of Life. Science Literacy - What Is Biodiversity? Video: The Oldest Animal Fossils. Possible fossil examples have been found in rocks that are around million years old, in Western Australia. Although commonly referred to as blue-green algae, cyanobacteria are not actually algae. Cyanobacteria, and bacteria in general, are prokaryotic life forms.
This characteristic is distinctive of bacteria and archaea; all other life forms on Earth, including real algae, consist of eukaryotic cells with organelles and with genetic material contained in one place the nucleus. Bacteria and archaea are hardy creatures. They thrive in hot, cold, salty, acidic and alkaline environments in which most eukaryotes would perish.
Despite this, they have a bad image: after all, bacteria cause many diseases in humans. An aid for students, a monitoring tool for teachers. Studying physics, biology, earth science and chemistry has never been so stimulating! Supplement the classroom lessons with those that Eniscuola has created for you with the teachers and students of Italian schools. Projects involving students from around the world, of different ages, which allow students to learn new content and release their energy, through participation and discussion.
A virtual thematic classroom, high school students linked from various parts of the world for 15 courses on the world of energy, organised by faculty of MIT in Boston and world-renowned experts.
We are a partner of the NECST project , the European Union programme that connects schools in Croatia, Holland, Norway and Italy in the creation of a digital platform for research and exchange of knowledge on energy production.
A storyboard full of texts and drawings to narrate one's country in an original manner: letting loose the imagination of children around the world in the edition. And for the winners, production of animated short films! Have you ever leafed through the pages of a book of Guinness World Records?
You can find records of all…. Can you be intelligent without a brain? The answer is "in a certain sense, yes" and this is demonstrated by…. There is a beetle that has been invading the Italian countryside and cities for some time now, a small insect….
World Sea Turtle Day is held on 16 June.
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