Human on the Tree of Life Click to enlarge image is a great way to explore the Tree of Life. Image:

Our family tree extends back for five to seven million years to the time when our ancestors took their first two-legged steps on the path toward becoming human. During this vast period of time our family tree grew to include many ancestors representing different species from our evolutionary past. Some of these species were our direct ancestors. Others were on side branches on our family tree and like ancestral ‘cousins’.

Sorting out our family tree

How do we know who our ancestors were and where they belong within our family tree?
There are two main stages involved in sorting out our ancestral family tree:

  1. recognising individuals and species that were human ancestors rather than other creatures.
  2. placing our ancestors into their correct positions within our family tree to reveal their relationships to our direct ancestors and our ancestral cousins.

Recognising our ancestors

Clues provided by fossilised teeth and bones have enabled scientists to identify many of our ancestors and to eliminate others from our ancestral family tree. If a fossil possesses human-like features, then it is recognised as a human ancestor that belongs somewhere within our extended family tree. For example, only humans and our immediate ancestors have:

  • premolar teeth with two cusps (raised bumps on the chewing surface),
  • a skeleton in which the base of the skull, the backbone, pelvis, legs, feet and toes are all constructed for walking on two legs.

Placing our ancestors into our family tree

Once a fossil has been identified as being a human ancestor, scientists try to determine where it belongs within our family tree. Its location will then reveal the relationships between it and our other ancestors.

Our direct ancestors are placed on the branches of our family tree that lead directly to our own branch at the top of the tree. Other ancestors have evolved in slightly different ways and are like ancestral cousins with a more indirect relationship to modern humans. These indirect ancestors belong on side branches of our family tree.

To place our ancestors in our family tree, scientists:

  • date fossils to work out when each ancestral species lived,
  • examine significant features of the fossils to determine the particular branch of our family tree to which they belong.

Sorting our ancestors by fossil age

Fossils are dated using one or more dating techniques that tell us how long ago they lived. This enables each fossil and species to be sorted along a time scale. Often, two or more species of human ancestor existed together at the same time. If this occurs, then each of these species is considered to belong on a different branch of the family tree.

Sorting our ancestors using their features

To determine the relationships between our various ancestors, scientists examine evidence about their physical and cultural features to determine the particular branch of our family tree to which they belong.

When sorting fossils according to their features, scientists are particularly interested in identifying features that are ‘primitive’ (arose early in our evolutionary past) and features that are ‘derived’ (evolved more recently). For example:

  • walking on two legs is a primitive feature that arose 5–7 million years ago. This feature developed early in our evolutionary past and is shared by all members of our family tree.
  • a large brain is a derived feature that only started to develop over the last two million years in some of our more recent ancestors.

Different species tend to be closely related if they share many of the same derived features. These species are therefore placed close to one another within our family tree. On the other hand, species that have fewer derived features in common are more distantly related. These species are therefore placed further apart in a family tree.

Species that are placed on different branches of our family tree are ones that have evolved in different ways and possess different sets of derived features. Although these species have evolved in contrasting ways and may have somewhat different appearances, they are still indirectly related to one another through their shared ancestors.

Sorting ancestors by their DNA

Is it possible to take DNA from ancient bones and use it to identify a species? Generally, the answer is no. DNA breaks down over time and in most cases either no longers exists in the sample or fragments are too small to be identified. However, if the bone or sample is young enough (usually no more than 60,000 years old), then DNA can be extracted. Great care is needed to ensure the sample isn't contaminated with modern human or bacterial DNA.

Extracting and producing viable DNA to study does not mean that scientists can then work out what species the bone or sample came from - unless they already have a DNA record on hand to compare it to. At the moment, the only human DNA to be extracted and analysed is that from Homo sapiens and Homo neanderthalensis.

An interesting case making headlines in 2010 was the discovery of a finger bone and tooth from Denisova cave in Russia. The bones were found in 2008 and date to about 30,000-50,000 years old. Mitochondrial DNA (mtDNA) was extracted from the remains, and then sequenced. The result was that the mtDNA did not match either modern human or Neanderthal mtDNA.

Little else could be gleaned from these studies so scientists started work on extracting nuclear DNA. This produced far more information. The 'Denisovians', as they have been nicknamed, were more closely related to Neanderthals than modern humans. This suggests the Neanderthals and 'Denisovans' shared a common ancestor after modern humans and Neanderthals split. Perhaps this ancestor left Africa half a million years ago with the Neanderthals spreading west to the Near East and Europe while the Denisovans headed east. However, this does not necessarily mean they are a 'new' species as they may be already known from fossils that have no DNA record to compare, such as Homo heidelbergensis or H. antecessor.

Revising our family tree

There are gaps in our ancestral family tree. These gaps often represent missing links – ancestors that we know must have existed but who remain to be discovered. However, as the search continues to find out about our past, these gaps continue to be filled in.

There are also different interpretations of the evidence currently available. Different interpretations have resulted in slightly different evolutionary trees.

Sometimes the discovery of a new fossil or the development of better technology provides new evidence. This evidence may alter present-day understandings and can therefore lead to revisions of our family tree but these revisions are an essential part of the quest to learn about our past.