If we exclude unicellular animals and those without differentiated organs or organelles, then for the most part they are marine animals that are invertebrates or fish. Despite having survived millions of years virtually unchanged, many of these species are now endangered due to human activity.
This is the most primitive of the multicellular organisms and the only living example of Placozoa. It has only a few types of cells and is believed to be a link between the unicellular organisms and more advanced multicellular ones.
The sponge family are extremely old – some estimate them to be 760 million years old. Although existing species today are not the same as those found in the fossil record, many are very similar. Sponges are extremely simple, sessile animals. It is believed that some sponges may have eventually given rise to other multicellular organisms, including possibly humanity’s ancestors.
Jellyfish have been around over 500 million years. Although some species have changed, many remain extremely simple animals that are not that different from their ancestors. Because of their soft bodies, finding fossilized remains of jellyfish is difficult and it is possible they are even older.
Horseshoe crabs are often called living fossils, remaining virtually unchanged 445 million years later. They have an extremely well developed immune system and this is believed to be at least part of the reason they have survived so well. Medical research has focused on them in an attempt to understand new ways of fighting off bacteria.
The coelacanth was believed to be extinct for millions of years until one was found in 1938 off the coast of South Africa. A few have been caught since then. Coelacanths have thick bony scales like extinct fish species. They can live 60 years, reaching a length of seven feet and a weight of 200 lbs.
This animal is over 500 million years old as a species. Like the horseshoe crab, it has survived the extinction of many other species. The nautilus is a species of cephalopod, making it related to squid and octopi. They are long lived but little is known about them in the wild. They are likely threatened by habitat destruction.
Lampreys are jawless fish. They are considered extremely primitive and virtually unchanged in the last 360 million years. They continue to thrive as a species today and have moved into new territories, creating a nuisance as an invasive species. They are currently in the news as their genome has been decoded and some researchers are looking at them for research into overcoming paralysis
This group of fish species has been around for more than 200 million years. Many of them are currently facing extinction. They can live up to 100 years and produce many eggs but few survive and their value as caviar adds humans to the predatory list.
Primitive species offer a glimpse into the roots of many other species and evolution itself. They also hold secrets that have allowed them to survive millions of years unchanged despite the many changes that have occurred in the world around them.
This means you're free to copy, share and adapt any parts (or all) of the text in the article, as long as you give appropriate credit and provide a link/reference to this page.
That is it. You don't need our permission to copy the article; just include a link/reference back to this page. You can use it freely (with some kind of link), and we're also okay with people reprinting in publications like books, blogs, newsletters, course-material, papers, wikipedia and presentations (with clear attribution).