What fossil skull is being unearthed right now while you are reading this?
The studies of Paleoanthropology and Archaeology are dynamic not static. These are very exciting fields that promise rich rewards of discovery. What could possibly be better than to be on a dig and unearth for the first time previously unknown fossils and artifacts. To hold in your hands a fossil skull from a species previously unknown to the world has to be one of the biggest thrills in science. How many more early man artifacts are out there yet to be discovered?
Judging by the last few decades there are many treasures that await to be unearthed adding significant knowledge about our past.
This essay on human evolution is not intended to be a detailed scientific treatise, rather it is an attempt to give an overview of how Homo sapiens evolved and the different steps we have taken along the way. The information here has been largely gathered from the most recent Paleoanthropological fossil finds that have been discovered and from the re-examination of previous finds from the last 150 years.
What I am going to talk about in this essay could change tomorrow, or even later today. There is a tendency to think of the ancient past as more of a static study but that view is far from the truth. It seems that new information and analysis is constantly updating, adding to and helping to reinterpret the past.
New finds are expanding our understanding of early hominins and how they evolved. The wealth of information scientists gathered in the last few years alone using highly sophisticated analytical tools has been truly enlightening.
Astonishing New Hominin Finds
In 2003 Homo floresiensis, a previously unknown hominin species was discovered on the Indonesian island of Flores. This new species of hominin has nicknamed ‘The Hobbit’ for its diminutive size.
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.
In 2010 Homo denisova or Denisovan, another previously unknown hominin species was discovered in a cave in Siberia known from the discovery of an ancient tooth.
In 2013 Homo naledi, yet another previously unknown hominin species was discovered in the Rising Star cave system of South Africa.
The past doesn’t change but our understanding of it does through new discoveries and new analytical technologies.
I think there is a tendency of human nature to take a snapshot of the world. In fact, I would call it the ‘snapshot mentality’. We mature to a certain level in our young lives and we look around and take a ‘snapshot’ of our interpretation of what we see and how the world works. We have a strong attachment to the snapshot we’ve taken and can hold on it for a long time, maybe the rest of our lives. When new events take place or new information comes in we look at it through the lens of the snapshot we’ve taken, sometimes conveniently nudging it here and there to fit in.
IMO this snapshot mentality causes stress on the system. Because if there is one thing we know it is that everything always changes. The more you try to hold on to a static view of the world the more real events and new information challenge that view and if you do not accept changes then stress will build up. There is no growth when something remains static.
The past is the past and what happened did indeed happen. Nothing could be truer. However, even looking at our own past the interpretation of it is seen through our own prejudices of the events and the past might look quite different from another’s point of view of the same events.
What doesn’t change are the events that took place but what can and often does change is the interpretation of those events especially based on new information that comes in about those events.
This is even truer when studying the ancient past. Simply put Paleoanthropology and Archeology is the scientific study of the evolution of mankind and its past cultures. This study is based on the fossil evidence and artifacts we have unearthed so far plus the genetic analysis of DNA.
The challenge for Paleoanthropology in studying the human lineage has always been the actual evidence we can hold in our hands for the various species is at best scant. 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 of 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.
The bulk of fossils we have are from 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. Or the even fewer fossil remains found for Denisova which will fit in the palm of one hand.
The picture I am painting here is about the very few actual physical pieces of bone you can 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.
What’s the problem you might think, just take the DNA out of the bones and sequence them and find out all kinds of things. Easier said than done.
Extracting DNA from fossil remains is extremely hard to begin with, 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 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 the predecessors of early humans and Neanderthals diverged as far back as 550,000 to 765,000 years ago.
To analyze DNA you first have to have the fossil bone fragments in hand which contain the DNA.
Anthropologists continue to find new fossils from our early ancestors and in fact continue to find altogether new 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.
We have come a long way in a short amount of time.
The missing link.
Charles Darwin caused quite a stir in his day. The thought that a human ancestor could be a monkey drove a lot of people crazy. It stills does in certain circles, mostly religious ones. In 1925, the ‘Scopes Monkey Trial’ took place in Tennessee. The trial was about teaching evolution in the public school system. Fundamentalist Christians argued the word of God as revealed in the Bible took priority over all human knowledge. Modernists Christians argued that evolution was not inconsistent with religion. This trial gained international attention at the time.
I bring this up as a reminder that at one time there was a tremendous amount of resistance to the theory of evolution. The pioneers of paleoanthropology were truly breaking new ground with their work.
Darwin published his work, ‘On The Origin Of Species’ in 1859. Soon after the question became who or what was the missing link. After all most people took the theory to mean that once there were monkeys and now there are humans so somewhere in between there was a missing link that bridged the gap.
William King in 1863 at the British Association for the Advancement of Science’s annual meeting became the first person to name a species closely related to man from fossil evidence. He had examined some skeletal remains found in the Neander Valley in Germany. He called the extinct species Homo neanderthalensis. And we were off and running.
In 1886 Eugene Dubois a Dutch anatomist organized the first expedition solely aimed at finding the missing link. He went to the island of Java, Indonesia and in 1891 found his missing link and it was dubbed ‘Java Man’. Java man turned out to be Homo erectus.
Once the hunt was on the following years yielded find after find. One significant find was in 1924 when Raymond Dart discovered the first Australopithecine. This find eventually established Africa as the birth place of humanity.
The first Hominins
Let’s take a look at what we’ve discovered so far but where do we start? How far back should we go? I will say that the Australopiths are considered to be the official start of the human lineage. There are species that have been found and dated before the Australopithecines and may well be ancestral to them but you have to start somewhere and before the Australopithecines the species to be considered that might be in the human linage were the first to have been around after the split from the chimpanzee lineage. So there is a time frame here of few million years that covers the span between the split between the chimpanzee lineage and the Genus: Australopithecus.
I am going to get a little technical here for a minute before we look at all the species in the human lineage. Here is what the common taxonomic system for modern man looks like. This is pretty much what you learn in school;
- Kingdom – Animalia
- Phylum – Chordata
- Class – Mammalia
- Order – Primates
- Family – Hominidae
- Genus – Homo
- Species - sapiens
Unless you are in grad school and then this is what you see;
- Kingdom - Animalia
- Subkingdom - Eumetazoa
- Clade - Bilateria
- Clade - Nephrozoa
- Superphylum - Deuterostomia
- Phylum - Chordata
- Clade - Craniata
- Subphylum - Vertebrata
- Infraphylum - Gnathostomata
- Clade - Eugnathostomata
- Clade - Teleostomi
- Superclass - Tetrapoda
- Clade - Reptiliomorpha
- Clade - Amiota
- Clade - Synapsida
- Clade - Mammaliaformes
- Class - Mammalia
- Clade - Eutheria
- Infraclass - Placentalia
- Clade - Exafroplacentalia
- Magnorder- Boreoeutheria
- Superorder - Euarchontoglires
- Grandorder - Euarchonta
- Miorder - Primatomorpha
- Order - Primates
- Suborder - Haplorhini
- Infraorder - Simiformes
- Pavorder - Catarrhini
- Superfamily - Hominoidea
- Family - Hominidae
- Subfamily - Homininae
- Tribe - Hominini
- Genus - Homo
- Species - sapiens
- Subspecies - sapiens sapiens
What I want to point out here is typically the taxonomy we learn and talk about is Family: Hominidae then Genus: Homo. But when examining our ancient ancestors we need to look at it more definitively. The reason being that the great apes are in the family Hominidae and if we go back and start talking fossils we want to talk only about the specific species that are in the human linage and not any other such as the chimpanzee or gorilla. So really we want to start on the path to modern man right after the last split from any other lineage and that begins with the Tribe: Hominini. Any species belonging in the Tribe: Hominini can be called hominins. So simply put if you are a hominin you are a member of the human lineage exclusively. The last common ancestor that the human lineage had with another lineage was with the Tribe: Panini (chimpanzees) and they are believed to have parted ways around 7 million years ago.
We start at 7 million years ago.
Let’s take a look at what we’ve discovered so far but where do we start? I will say that with the Australopithecines there is a general consensus of opinion that they are at the start of the human lineage. There are species that have been found and dated before the Australopiths and may well be ancestral to them but there is not a general consensus about the classification of them. Before the Australopithecines the species to be considered that might be in the human lineage were the first to have been around after the split from the chimpanzee lineage. So, there is a time period here of few million years that covers the span between the split with the chimpanzee lineage and the Genus: Australopithecus.
The species before Australopithecines
The following four species are those considered that could very well be the first in the hominin line.
Sahelanthropus tchadensis at 7 million years ago is one of those species where you have a partial skull and just a few other bone fragments. However, it is thought that this species might be right at the split from the chimpanzees. It is also thought to be ancestor to tugenensis.
Orrorin tugenensis dating back to 6.2 to 6 million years ago. Scientists conclude that tugenensis has morphological traits that would allow it to be in the hominin lineage. It is also thought from these fossils that the split between the apes and humans took place between 7 and 9 million years ago.
Ardipithecus kadabba 5.6 million years ago is known only from a few teeth and small pieces of other bone. Most likely ancestor to ramidus.
Ardipithecus ramidus is another species who dates from 5.2 to 5.8 million years ago. Some scientists think this species could be a hominin but others think it belongs in the pan lineage (chimpanzees).
And So It Begins
Australopithecus anamensis is the first Australopithecine discovered (so far), dated 4 to 4.5 million years ago with 100 bone fragments found this is the first general consensus hominin we have (and when I say we I mean you and me).
It is not my intent in this paper to talk about each hominin species discovered. I have under the Human Evolution tab talked at length about several of the species but I just want to give an overview of the human tree here. So, with that in mind here are the hominin species. This list has 21 species. It could be only 15 species, 6 of them are questionable because of the few fossil remains found or reclassification. The reclassified or questionable are colored in red.
The next two are most likely A. afarensis but some think they deserve their own species designation.
The next three species are either classified as Australopithecus or a new designation of Paranthropus. These three species were formerly known as the robust Australopithecines.
The next three used to be considered separate species but now are considered by most to be variations of Homo erectus.
Due to just a handful of bone fragments unearthed (and some other stuff) I have taken it upon myself to declare this next one as early Homo heidelbergensis and not a separate species.
I would encourage everyone to google these names to find out all about each species. If you are like me it is totally fascinating reading and well worth the time. Wikipedia is a great source but I have to say I really like the Australian Museum’s site, they have very well written articles. Becoming Human is also a really nice site. There is also a wealth of information at the Smithsonian National Museum of Natural History in their section ‘What Does It Mean To Be Human’.
Something jumps out with even a casual look at the list above. Seven Homo species walked the earth during the same time period. They surely interacted (more on this in part four) and we know from DNA some interbreeding was going on… hubba hubba.
But what does it all mean?
Here’s the thing. We can talk all day about species this and that but really what does it mean? What is the path? What is the direct route from A to B or is there one? Well you are here right now reading this so we know there are dots to be connected.
One thing we know for certain is that some of the species are our direct ancestors and a few were dead ends, off shoots that went extinct.
Extinction is a natural occurring event of the evolutionary process.
While there are some animal species that have been around quite a long time (Nautilus 500 million years, Horseshoe crab 230 million years) mammals in general have a species ‘lifespan’ of about one million years to ten million years with most being in the lower end of that range.
Worldwide extinction level events have taken place in the earth’s history. The most commonly known one is the one that ‘wiped out’ the dinosaurs about 66 million years ago. About 75% of all the species on the planet became extinct resulting from an asteroid hitting the earth. There have been 5 major such extinction level events that we know of and some say over twenty other minor ones. There is a growing widespread consensus that we are in the mist of the 6th great extinction level event.
However, I am not going to talk about that aspect of extinction. I am going to talk about extinction of an individual hominin species and how in the course of normal evolution species go extinct.
The Extinction Vortex
I love the term ‘extinction vortex’. There are certain conditions that need to be met for the sustainability of any species. Once these conditions slip too much in a negative direction a species can start to circle and eventually go down the extinction vortex. Principle among the conditions for sustainability is population size. Once the population of any species reaches a critical low, that species will in effect collapse upon itself and cease to exist. And apropos to the vortex visual what happens is the pace towards extinction increases as the population decreases. The smaller the population gets the closer towards extinction it becomes.
Minimum Viable Population
There is a critical population size for a species and once that threshold number is reached there is no turning back. The term for that number is the ‘Minimum Viable Population’ (MVP). This minimum viable population size needs to be met in able for a species to sustain itself. Once a population gets to a certain low point (below the MVP) some environmental event will wipe it out. By environmental I mean it could be a drought, a volcano, climate change, a disease, the food supply shrinks, inbreeding because of the low population size or some other factor happens and because the species was at or below the minimum viable population it will not recover, there simply will not be enough individuals to keep the species going.
To maintain a genetically viable population for sustainability a species needs to have in excess of 5,000 individuals. Fall below that number and the species enters the extinction vortex.
The figure of 5,000 individuals assumes that the population is within a given geographical area so they can interact with each other.
We know that hominins travel and spread out. So what this means is if you say have 3K Homo heidelbergensis individuals in Africa and 2K in Europe and 3K in Eurasia, that distance is too far for them to interact and they will not have the numbers to sustain themselves.
There are several reasons why a species would decline in numbers. It could be from outside environmental factors like I have previously talked about or it could be from internal influences like mutations that affect the gene pool.
Of course the other main reason a species will become extinct is that it evolved into another one.
I think it is worth repeating that Extinction is a natural occurring event of the evolutionary process. Because with mutation, gene flow and natural selection an entire species will ‘evolve’ or change into another one. And this is what we are looking for when we want to connect the dots between the Australopithecines and Homo sapiens.
We are going to look at that and more in part four of this paper on Homo sapiens and Human Evolution.