{"id":13709,"date":"2023-10-17T13:39:46","date_gmt":"2023-10-17T02:39:46","guid":{"rendered":"https:\/\/rationalemagazine.com\/?p=13709"},"modified":"2023-10-17T13:39:46","modified_gmt":"2023-10-17T02:39:46","slug":"turning-back-the-clock-on-the-evolution-of-animals","status":"publish","type":"post","link":"https:\/\/rationalemagazine.com\/index.php\/2023\/10\/17\/turning-back-the-clock-on-the-evolution-of-animals\/","title":{"rendered":"Turning back the clock on the evolution of animals"},"content":{"rendered":"

There are estimated to be nearly\u00a08 million species of animals<\/a>\u00a0living today, making up the majority of Earth\u2019s documented biodiversity and inhabiting almost all of its environments. However, for most of Earth\u2019s history animals were completely absent.<\/p>\n

The date of the first animals marks a shift in the history of life on Earth. Of course, as animals ourselves, it\u2019s also the story of our origins. Without animals, our planet would have been a very different world.<\/p>\n

The question of exactly when animals first evolved has puzzled scientists for centuries.\u00a0Even Charles Darwin<\/a>\u00a0was stumped. He predicted a long history of evolution from simple single celled organisms to complex animals. However, the oldest animal fossils Darwin knew of, from around 500 million years ago, were large enough to be visible to the naked eye and often had shells and skeletons.<\/p>\n

My team\u2019s study<\/a>\u00a0hopes to help settle the debate through a new approach to the question.<\/p>\n

Scientists used to think animal fossils appeared suddenly in a time period around 500 million years ago called the\u00a0Cambrian Explosion<\/a>, so named because the fossil record seems to show a sudden boom in complex life at this time. We know the first animals evolved in the oceans, and with their abilities to move and burrow, they fundamentally altered the\u00a0Earth\u2019s carbon cycle<\/a>.<\/p>\n

However, since Darwin, palaeontologists have discovered thousands more fossils, some of them more ancient than the Cambrian Explosion. Impressions of strange-looking organisms, called the\u00a0Ediacara Biota<\/a>, were discovered in the 1950s in rocks and have been dated to around 574\u2013539 million years ago (the\u00a0Ediacaran Period<\/a>). Some of the Ediacara Biota fossils represent the\u00a0oldest animal fossils known<\/a>.<\/p>\n

Yet these recent advances have taken the animal fossil record back only so far. Reports of older and more simple animal-like fossils have been published. For example,\u00a0sponge-like fossils<\/a>\u00a0from the Mackenzie Mountains, Canada are around 800 million years old. But these older fossils\u00a0cannot yet be conclusively proven to be animals<\/a>. They could be algae or perhaps not even fossils at all.<\/p>\n

Also, the presence of fossils doesn\u2019t necessarily confirm the date for the evolutionary origin of animals. Only a fraction of life has ever been fossilised meaning the fossil record is\u00a0full of gaps<\/a>.<\/p>\n

Of course, as animals ourselves, it\u2019s also the story of our origins. Without animals, our planet would have been a very different world.<\/strong><\/p><\/blockquote>\n

With no definitive first animal fossils, palaeontologists have turned to molecular biology, using genetics to trace ancestry. This technique, called the\u00a0molecular clock<\/a>, works by sampling the genetics of modern animals and comparing their DNA. The differences in DNA between species shows how much evolution has happened.<\/p>\n

Although molecular clocks can only provide estimates for the timing of animal origins,\u00a0most converge on around 800\u2013700 million years ago<\/a>, long before the Ediacaran extent of the animal fossil record.<\/p>\n

This gives us two estimates for animal origins, more than 200 million years apart. On the one hand, fossils extend to 574 million years ago; on the other, molecular clocks suggest animals could be as old as 800 million years.<\/p>\n

In our\u00a0recent paper<\/a>, my colleagues and I suggest a new way to estimate the timing of animal origins. Instead of documenting the oldest animal fossils, we first considered which kind of rocks could preserve those animals.<\/p>\n

An animal\u2019s body type determines what kind of rock can fossilise it. Many animals have shells and skeletons that can be preserved in most types of\u00a0sedimentary rocks<\/a>\u00a0\u2013 such as sandstone \u2013 that start as sediments in rivers. However, the\u00a0first animals did not have shells or skeletons<\/a>. Animals evolved these structures at a later time.<\/p>\n

Studying deposits such as the\u00a0Burgess Shale<\/a>\u00a0fossil bed in Canada, which is around 508 million years old, showed my team that fossils of animals lacking shells or skeletons, such as worms, are often\u00a0confined to rocks that are rich in clay minerals<\/a>. Clays have\u00a0antibacterial properties<\/a>\u00a0and can\u00a0stop the decay<\/a>\u00a0of animal soft tissues.<\/p>\n

We tested rocks from geological eras older than the Ediaracan period (635 million years ago) to work out which ones had the clay-rich composition necessary to fossilise the first animals. Rare rocks around\u00a0790 million years old from Canada, Norway and Russia<\/a>\u00a0have the necessary clay-rich properties.<\/p>\n

Although these rocks could preserve the first animals, none of them do. This suggests that animal fossils are absent at this point in time, not because they couldn\u2019t be preserved but perhaps because they weren\u2019t there \u2013 that animals hadn\u2019t evolved yet.<\/p>\n

Palaeontologists now need to search more geological sites across the planet to confirm the youngest clay-rich rocks that could have preserved the first animals, but where animal fossils are absent. This will help us to home in on the true timing of animal origins.<\/p>\n

This article was originally published in <\/strong><\/em>The Conversation<\/strong><\/a>.<\/strong><\/em><\/p>\n

Photo by Wes Warren<\/a> on Unsplash.<\/strong><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

There are estimated to be nearly\u00a08 million species of animals\u00a0living today, making up the majority of Earth\u2019s documented biodiversity and<\/p>\n","protected":false},"author":692,"featured_media":13710,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[65],"tags":[631,513,632],"coauthors":[633],"class_list":["post-13709","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-science-health","tag-animals","tag-evolution","tag-palaeontology"],"acf":[],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/posts\/13709","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/users\/692"}],"replies":[{"embeddable":true,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/comments?post=13709"}],"version-history":[{"count":1,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/posts\/13709\/revisions"}],"predecessor-version":[{"id":13711,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/posts\/13709\/revisions\/13711"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/media\/13710"}],"wp:attachment":[{"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/media?parent=13709"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/categories?post=13709"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/tags?post=13709"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/rationalemagazine.com\/index.php\/wp-json\/wp\/v2\/coauthors?post=13709"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}