Fossils are concrete evidence of our picture of life on earth.
Knowing when and where the oldest fossils of different species appeared provides details about the evolutionary tree.
But while fossils are locked in ancient rocks, the fossil record is far from set in stone.
A new footprint, a newly uncovered outcrop — even a fresh look at some ancient bones that have been in a drawer for decades — can shake the dates and branches of the evolutionary tree.
Here are a dozen of our current "oldest known" fossils, discovered - or best aged - within the last 20 years.
It is not an exhaustive list of our oldest known organisms, just a few key points on the evolutionary journey.
With so many stones to turn, the fossil record - and the list of such record holders - will continue to evolve.
The "oldest known" fossil in the world: 3.7 billion years old
Scientists announce what may be the world's oldest fossils— Ruins of ancient bacterial colonies known as stromatolites in 3.7-billion-year-old rocks in Greenland.
That's a cool 200 million years ahead of the 3.48 billion year old fossil stromatolites from the Pilbara region of northwestern Australia.
Molecular clocks — the mutation rate of genetic material — suggest life on Earth began about 4 billion years ago, so fossils as old as Greenland stromatolites suggest life on Earth may have been very early has begun.
Multicellular life: 1.56 billion years
Algae-like fossils found in 1.56-million-year-old rocks in China are the first known examples of larger organisms made up of many cells -- the beginnings of complex life.
Discovery,reported earlier this year, places organisms at the center of what was once known as “the boring billion.”
Until this discovery, no large multicellular life forms were seen in the fossil record until 600 million years ago, so this was a huge discovery.
First known fish: 505 million years ago
These 505-million-year-old fossils from Canada aren't just the oldest known fish fossils — they contain incredible detail of the animal's facial features.
the discovery ofMetaspriggina,Published in Nature in 2014, was remarkable because the earliest fish fossils from the Cambrian period are very rare and poorly preserved.
But these beautiful fossils have camera-like eyes and well-defined nasal structures, and a series of exceptionally well-preserved arches near the front of their bodies that are early evidence of jaws.
Earth organism: 440 million years old
The beautifully preserved filaments of a fungus that lived 440 million years ago are the oldest known fossils of a unique terrestrial organism.
The pioneer known as Tortotube,helped lay the foundation for more complex organisms to take root and thrive on Earthsaid Dr. Martin Smith of Durham University.
"Tortotube's tiny threads would have helped consolidate and hold the first floors together, allowing deeper floors to form," said Dr. blacksmith
"Tortotube would also have broken down organic matter into a form in which its nutrients were available to other organisms, and provided some form of fertilizer for the first land plants."
Reptile: 318 million years old
Provide fossil footprints in a 318-million-year-old dry riverbed in present-day Canadathe oldest known evidence of reptiles.
"The footprints are from the Carboniferous period, when a single supercontinent (Pangaea) ruled the world," said Professor Mike Benton of the University of Bristol.
The footprints may have belonged to a small lizard-like animalHylononus lyelli. SSlightly younger skeletons of this creature (only 312 million years old) were found in neighboring Nova Scotia in 1859.
Oldest known dinosaur: 245 million years old
The fossilized dinosaur bones discovered in Tanzania may be the oldest dinosaur — or dinosaur relative — discovered to date.
But they're far from a recent discovery: the bones were discovered in 1930, but they stayed in a box for decades until they wereRe-examined in 2012 using modern scanning techniques.
The scans revealed the nicknamed dinosaurNyasasaurus parringtoni, lived 240 million years ago to 245 million years ago, when the planet's continents were still joined to form the landmass Pangea.
Placental mammal: 160 million years ago
About 90 percent of today's mammals, including us, are placental mammals. We give birth to fully developed children who were raised in the womb.
The earliest evidence we have of this group was across the forest floor of present-day China 160 million years ago.
In 2011, Chinese scientists discovered the fossil remains of a small animal resembling a mouse or shrew and named itJuramala-Synesis.
Two years after its discovery, another equally old fossil of a mouse-like mammalEurasian Rugosodonwas discovered - also in China.
These surpass the previous holder of the title of oldest known placental mammal - again from China - a squirrel-like animal dating back 125 million years ago.
Modern bird: 130 million years old
For many years the iconic 150 million yearsArchaeopteryx-Fossil— the distorted skeleton of a small, feathered animal with a long tail and claws — thought to be the first bird, but it really sits somewhere between dinosaurs and modern birds.
Today's birds belong to the Ornithuromorpha lineage.
In 2013, scientists in China discovered two beautifully preserved fossils from this group.
The fossil remains belonged to a previously unknown species of bird that lived 130 million years ago.
Chamado Archaeornithura meemannae,Fossils have pushed back the oldest known modern bird by about five million years.
Flowering plant: 130 million years old
This is the fossil of a plant that grew in swamps 130 million years agoearliest example of a flowering plant, scientists reported in 2015.
Montsechia vidalii, once grew abundantly in freshwater lakes in what is now mountainous Spain.
The fossil outperformed another aquatic plant,Archaefrutus sinensis, who lived in modern-day China for five million years.
"Based on this new analysis, we now know that Montsechia is contemporary, if not older, than Archaefructus," said Dr. David Dilcher.
Primate: 55 million years old
The nearly complete skeleton of a tiny tree-dweller that lived 55 million years ago is the oldest known primate - the ancestor of all humans, great apes and great apes.
Archicebus Achillalso known as the "first long-tailed monkey", was discovered in China in 2003 by a farmer in a quarry near Jingzhou.
It took another 10 years of painstaking work to describe in detail the fossil locked in rock.
"Archicebus is radically different from any other primate, living or fossil, known to science.said co-researcher Chris Beard of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
First prehistoric man: 2.8 million years ago
The human ancestral group to which we belong is calledHomo.
Until recently it was believed that a species known asa skillful man- also known as Handy Man - was the first member of our group.
But the discovery of a2.8 million year old partial jaw in Africa2015 shook the evolutionary pedigree.
The lower jaw predates 400,000 yearsa skillful man, still has characteristics of both Handy Man and a more primitive group of human ancestors called Australopithicus, which includes the famous onesfossile Lucy.
olderA wise man:195,000 years
Finally - the first evidence of anatomically modern humans is on the scene!
Omo I and Omo II are the oldest known fossils froma wise man– the kind that finally emergedHomo sapiens sapiensor us.
When the bones were discovered in Ethiopia in 1967, their age was estimated at 130,000 years. But in 2005, after thatre-analyzing the date of the sediment in which the bones were foundscientists have reduced the age of the bones to 195,000 years, earning them the title of oldest knowna wise manFossils, at least for now...