Human Fossils, An Introductory Guide

 

Homo heidelbergensis skull
Heidelberg Man 1.2 million to 200,000 YA

What are human fossils, how are they created and how are they dated? In my last post I wrote about the very beginnings of our human legacy.  The story can only be revealed through study of fossil remains and as I pointed out there are very precious few fossil remains in the beginning of our history. So few that you have wonder how can anyone deduce very much from them.

A large part of deciphering and sorting out of discoveries is being able to do comparative analysis from the cumulative body of previous finds.  A tremendous number of insights can be gained from the smallest evidence. 

The way a single bone or bone fragment is shaped will reveal how that bone is connected to others and what that means.  For example, the road to becoming bipedal, walking on two legs, is understood by how the skeletal remains evolve more and more towards the first fully upright walking human, Homo erectus. (Homo erectus means ‘upright man’). From a comparative analysis of skeletal remains of hominins through millions of years a single bone can indicate evolutionary advances.  A solitary foot bone can point towards how bipedal a species would have been.

I am going to talk more about this in my next post when I go over the Australopiths.  The genus of the Australopithecines are the precursors to the genus Homo which includes us, Homo sapiens.  But for now, after the introductory post on the human legacy, I think it’s a good time to understand what exact a fossil is, how they are created and how they are assigned a date.

fossil fish

What Are Fossils?

The legacy of life on earth is revealed to us through fossils.  It's really hard to imagine what our understanding of the history of like on earth would be like without fossils to enlighten us.  We only know about our human legacy through the study of hominin fossil remains that have been discovered. From the first accidental finds of Neanderthal bone fragments to the sophisticated analysis of DNA contained in those fossilized bones and others from the cache of fossils we currently have, the constant is the fossil itself. What a great gift fossilization has been.

Dead Men Do Tell Tales

The dead do indeed speak and fossils are the language they use.  Speaking the language of 'fossil' is what Paleoanthropologists do.  Paleoanthropology is the science of discovery and study of fossils in the human lineage. So, what exactly is a fossil and how are the formed?

Scientists have put together a good overview of the history of like on earth but it is far from complete. There are stringent conditions that have to be met for a living being to become a fossil.

Hominin fossils are very rare and precious artifacts as we have seen from my last post introducing the human legacy. In fact, ancient human fossils are the most rare and treasured artifacts in the world.

A handful of early hominin fossilized bone fragments are all we have at the moment to go on.  New discoveries happen all the time and who knows, maybe today someone will unearth a great new find shedding more light on the human legacy.

Wait a minute, you might be thinking.  There are fossils all over the place, they seem quite common.  But the truth is of all the quintillions of plants and animals that have lived on earth only a very small few have been fossilized for us to find.  No doubt there have been millions if not tens of  millions of different species that have lived and died, leaving no fossil record at all.

How Bones Turn Into Fossils

Whether or not an organism becomes fossilized depends mostly on the environment surrounding it when it died.  Deserts and arid caves are good. Being entombed by a cave-in or rockslide is good also.  Tar pits have been good preservers of bones.  Basically, what is good is to have the organism encased away from the elements (water and oxygen) as much as possible and as soon as possible.

Petrified bone is the main way we think of fossils.  The process plants and animals go through to become petrified is called 'Petrifaction' and petrifaction is accomplished mostly by permineralization. When you see fossils in museums and they are almost certainly formed by permineralization.

Life After Death Is Risky

Not all bones fossilize equally well.  Hominin bones with a lot of surface area, lightweight bones and small bones have a much less chance of becoming fossils than teeth, jaws, skulls and the bigger leg and arm bones.  Teeth actually have the best chance.  Rib bones while actually pretty big are more porous so they don't fossilize well.  Smaller bones are also more likely to get crushed or washed away or otherwise lost to time.

Most fossils are found in sedimentary rocks.  Sedimentary rocks are formed, as the name would imply, from sediments of material at the earth's surface.  This sediment material is made by erosion and weathering then moved to a place by wind or water and settled into a process called sedimentation.  Sedimentation can also occur from minerals and even shells of creatures that will settle out of suspension from water.

So, one likely scenario for fossil creation is an animal perishes from drowning, or a flood or quickly after death is covered up by sediments, the water recedes and then through time the bones are fossilized as the sediments turn into sedimentary rock.

I was lucky enough to visit Dinosaur National Monument in the summer of 18.  Apparently there was a big flood and dozens of dinosaurs got caught up in it.  There is a big wall and visitors can see over 1,500 dinosaur fossils exposed on the cliff face inside the Quarry Exhibit Hall.  Most are from the really big dinosaurs too.

Here is a photo I took of the great wall of fossil dinosaur bones, it is a very amazing sight..

wall of dinosaur bones

And another one showing a closer view of  fossilized bones.

Types of Fossilization

Most people think of fossils as bones but fossils can be any trace of a living ancient organism.  A fossil could be a footprint, a cast of a bone, even soft body parts can be preserved under the optimal conditions, i.e. frozen or encased in amber as shown in the next photo.

                             amber fossil of dragon fly

Trace fossils are basically the traces that a living being has left behind to indicate its existence.  These trace elements can be tracks or footprints, nests, eggs, excrement (coprolites) and even bitemarks or claw marks.

An example of a fossil footprint from the Triassic period in the next photo.

a trace fossil

A cast fossil in one that leaves a cast of replica of the organism.

cast fossil of horseshoe crab

Chemical fossils are substances like fossil fuels.  They are formed by the buildup of organic matter under high pressure and temperature coupled with the action of bacteria.

A subfossil is classified as a fossil not yet completed.  A once living being that may or may not actually complete the fossilization process.

There is also a category of fossil called a Living Fossil.  These are animals that still live but have changed little in millions of years.  The Chambered Nautilus has been around for 500 million years and the Horseshoe Crab has been around for 445 million years.  Both of these species have been basically unchanged since long before the dinosaurs.

chambered nautilus

But mostly when we talk about hominin fossils we are talking about bone, which when fossilized can last millions of years until exposed to the elements again.  Once exposed these fossils need to be treated very soon to preserve them.

If you have ever seen an episode of 'Dino Hunter' on the Discovery Channel you will really see how important it is to preserve a fossil find immediately.

fossil of t-rex skull

The Study of Human Fossils

The challenge for Paleoanthropology in studying the human lineage has always been the actual evidence we can hold in our hands from the various species is unfortunately minimal.  With some hominin species we are talking a dozen or so fossil bone fragments.

To date there have been approximately 6,000 fossil bone fragments found from all the species in the human lineage other than Homo Sapiens.

The fossil remains unearthed are mostly fragments or single bones or isolated teeth.  Complete fossil skulls and skeletons are almost nonexistent.

Here is a photo of  'Lucy' (3.2 million years ago), the most complete fossil we have of an Australopith. It is only 40% complete.

fossil of Lucy 3.2 million years ago

The bulk of fossils we have are from Neanderthals ( Homo neanderthalensis), which there have been bone fragments from 400 individuals found including about 20 complete skeletons, and the over 1,500 bone fragments from Homo naledi discovered in 2013.  Contrast that with the 14 bone fragments found from what some scientists want to call a separate species - Homo antecessor.

In 2008 a new species of Australopith was found.  Australopithecus sediba was found in South Africa.  There have been 220 bone fragments found so far with this new species.  Some of fossils shown in the next photo.


Australopithecus sediba fossil

In a separate find from 2008, there were a few teeth and a part of a finger bone found in a cave in Siberia.  And those few samples plus a jaw bone found in Tibet in 2019 are the sum total we have for the Denisovan species.  The jaw bone is shown in the next photo.

jaw bone of a Denisovan

The picture I am painting here is about the very few actual physical pieces of bone you hold in your hands.  The tremendous work that has been done by scientists with this amount of material has been astounding to say the least.

DNA and Fossils

What's the problem you might think, just take the DNA out of the bones and sequence them and kind out all kinds of things,  Easier said than done.

To analyze DNA you first have to have the fossil bone fragments in hand which contain the DNA.  Then extracting that DNA from the fossil is extremely hard to begin with and actually getting your hands on enough to analyze is tough indeed.

DNA breaks down rather quickly.  Its estimated half-life is 521 years.  To find enough DNA to work with after hundreds of thousands of years is really, really hard.  Immediately after death the enzymes in DNA start to collapse which break down the bonds that form the backbone of DNA.  Then there are the environmental factors like water, temperature, microbes, oxygenation and the pressure of the earth upon the bones from being buried.  Groundwater seeping down into the earth over thousands of years is considered to be the most damaging factor.  Contamination from extracting the fossils in the first place and even from studying them are an issue also.

Having said all that in 2016 a published report reveals the sequencing of mitochondrial and nuclear DNA from a 430,000 year old Neanderthal.   Based on this Neanderthal DNA it now appears that early humans and Neanderthals diverged as far back as 550,000 to 765,000 years ago.

In yet another twist, there is some new data that speculates archaic Homo sapiens might be 800,000 to a million years old.

What Can We Learn From Fossils?

Anthropologists continue to find new fossils from our early ancestors and in fact continue to find altogether new hominin species.  The techniques and tools to study their DNA keep advancing.  All of this does not alter the past in any way but it does change our understanding of it.  There are still some missing gaps that need to be filled but the big picture is almost complete.

Some of the things we learn from hominin fossils.

  • Evolutionary processes and trends
  • relatedness to other species
  • body shapes and sizes
  • species type
  • physical capabilities 
  • growth patterns
  • like expectancies
  • pathologies 
  • inferences about size and shape of muscles
  • understanding locomotion
  • Positioning of eyes, ears, nose on the skull
  • gender
  • age of individual
  • age of species
  • diet
  • injury
  • diseases 
  • how quickly children grew
  • even some social behaviors can be determined
  • brain size

How Fossils Are Dated

In order for us to better understand the evolution of the human legacy we need to be able to put a date on the fossils we find so we can see where a particular fossil will fit in the timeline.  Scientists use a variety of techniques to date geologic formations and strata.  The same is true for Paleoanthropologists when dating hominin fossils.

All things considered, the accuracy of fossil dating decreases with the age of the fossil, the older the fossil the harder it is to pin down an exact date.  Nevertheless, there are several techniques that can be used to determine age.

The two methods of dating are the relative and absolute.  Fossil sites often employ both methods when dating fossils.

Relative dating uses the concept that the deeper layers of the earth will be older than the layers above them.  Estimates for the age of fossils can be made from knowing the age of the layer of earth it was found in.

Radiometric or radioactive dating is the main method for dating fossils.  Radiometric dating uses the half-life of isotopes in carbon, uranium and potassium.

The most commonly known radiometric dating is carbon 14 dating.  Archaeologists use carbon 14 dating quite a bit but for Paleontology and Paleoanthropology the half-life is too short.  The half-life of carbon isotopes is 5,730 years.  What that means is every 5,730 years half of the unstable isotopes of carbon become stable, a measurable effect.  The oldest reliable date you can get with carbon 14 is 50,000 years and that is not nearly old enough for dating the human lineage not to mention other animals like dinosaurs, however, it is quite useful for Archeologists to date ancient cultural sites. 
Uranium and potassium have a half-life of over a million years allowing for the use in dating of older artifacts.  But how is this accomplished?

Fossils and the sedimentary rocks fossils are formed in do not contain uranium or potassium.  However, igneous rock or rock made from cooled magma do contain uranium and or potassium. 

volcano erupting

Conveniently, volcanic ash contains igneous rock.  Considering the volcanic activity of the earth’s history there are layers of volcanic ash laid down at regular intervals. And the best part is you don’t need a thick layer at all, minuscule particles will do the trick.

This is how it works.  Scientists use a technique called bracketing to determine the age of a sedimentary rock layer.  They age the layer right below the sedimentary rock and right above it.   Using this bracketing method scientists have recorded the age of rock layers around the world, so when a fossil is found it can dated by knowing the age of the rock layer it is found in. 

There are a few other dating methods used to augment the above.

Luminescence dating which includes optically stimulated luminescence, infrared stimulated luminescence and thermoluminescence dating.  All of which use exposure of mineral grains to sunlight and heat to determine age.

Analyzing amino acids, using tree rings and even using the magnetic shifts of the earth’s poles can be used in some cases to help determine the age of a fossil.

When we are talking about the human legacy comparative analysis between the known ages of fossils is also a very useful tool in fossil age determination.  Using all the techniques available we presently we have established a really good idea of the timeline for hominins.  

Palaeoproteomics

Since the onset of the digital age, scientists have been able to refine all previous dating techniques resulting in the timeline of fossil remains becoming more and more accurate.  One new technique highly applicable to hominins is Palaeoproteomics.

In Palaeoproteomics scientists analyze ancient proteins found from teeth and other remains. So far scientists have recovered proteins from 1.8-million-year-old animal teeth and a 3.8-million-year-old eggshell. Now, they hope that palaeoproteomics could be used to provide insights about ancient hominin fossils that have lost all traces of DNA.

Summery

In the study of man, the fossil history is the study of bones and other associated artifacts.  The understanding of man through the study of fossils is the same as the understanding of life on earth through the study of fossils.   There are pieces missing, some species are underrepresented but at this point we have a really good understanding of what happened and the timeframe for it.  

The vast majority of hominin fossils have been found in Europe, parts of Asia and Africa. There are still large parts of Africa, Eurasia and Asia that for a variety of reasons (mostly political) have not been explored for hominin fossils.  So, it is possible, especially given the recent finds in the past two decades, that different species of hominin are yet to be discovered.  Even searching in the same areas that have yielded fossils before can still be productive.

There have been enough fossil remains found to fill out a big picture of human evolution.  It would be nice to see more discoveries of transitional forms to help connect the dots.  However, transitional periods take place over a shorter timeframe which offers a decreased chance for leaving a recoverable example.  

However, speaking of new discoveries.  Two new hominin species have bee discovered and reported this year, 2021.

Homo longi or "Dragon Man." This new species was actually discovered in 1933 and this year, 2021, under went a re-evaluation. It was determined to be a completely new hominin species, closer to modern man than Neanderthals.


And if that wasn’t enough another totally new species discovered in June of 2021 dubbed Nesher Ramla Homo because it was found at the Israeli site of Nesher Ramla. This individual falls right smack in the middle of Neanderthals and modern man: a unique population of Homo never before recognised by science. 

Nesher Ramla Homo fossils

Wow.  Two new species of hominins found in one year and both are really helping to fill out our understanding of the human legacy.  Today or tomorrow yet another discovery could be made that will amaze us all, however, when that happens, we will be adequately equipped to feel confident enough in the dating of any new discovery using all the tools we have available to us right now. 



 


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