Prehuman fossils: an embarrassment of riches

Introduction

One of the issues most frequently raised by both creationist and and intelligent design writers is the question of gaps in the fossil record, and, in particular, of “missing links” between “apes” and humans. In one sense, such a question is improperly posed — science does not propose that a modern “ape” (gorilla, bonobo or chimpanzee) changed into a human, either quickly or slowly, but instead that gorillas, bonobos, chimpanzees and humans all had a common primate ancestor, probably about 10 million years ago.

In any event, the question of whether there are “missing links” between ancient primates and humans is now rather pointless, given the many fossils that have been discovered, representing numerous different species. The only real questions here are how all of these ancient fossils fit in the family tree, which are in the direct line of human ancestry, and how many more species are still to be found.

To appreciate how far the study of prehuman fossils has come, even in the 1990s it was generally thought that the first hominins (the group that includes modern humans and their extinct predecessors) appeared about four million years ago, and then descended in a fairly “linear” fashion to Homo ancestors who left Africa about one million years ago, some of which subsequently became the Neanderthals, a relatively brutish and not-very-smart species who were, in turn, replaced by Homo sapiens about 30,000-50,000 years ago.

A new, more complicated family tree

But we now know that this picture was too simplistic, and even downright wrong in some particulars. For example, researchers now recognize that Neanderthals were actually quite sophisticated. The full history of prehuman species is significantly more complicated, with numerous branches that may or may not have led to modern humans. What’s more, emerging evidence has shown that there were more of our “cousins” living on the planet at the time of Neanderthals. A branch known as Denisovans has been discovered that, based on DNA analysis, is distinct from both human and Neanderthal lineage, yet some of its DNA lives on today in human DNA. Yet another branch is Homo floresiensis, popularly known as the “hobbit” fossil, which may have survived until as recently as 18,000 BCE (although there is still debate on whether or not it is a non-human specimen). Recent updates on these developments are available in two Scientific American articles [Harmon2013; Wood2014].

Here is a “family tree” graphic, from the September 2014 Scientific American:



Creationists’ classification of prehuman fossils

Creationist writers, who deny that there are any “transitional” fossils between primates and humans (because this would counter their belief in “special creation” of humans), typically deal with prehuman fossils by classifying them as either “ape” or “human.” However, in a rather ironic twist of events, these writers have not been able to agree as to which fossils should be classified “ape” or “human.” Here is a chart of creationist classification of some prehuman fossil specimens, based on a similar chart in [TalkOrigin2009]:

Creationists’ Classifications of Prehuman Fossils

Specimen Brain cavity
size
Cuozzo1988 Gish1985 Mehlert1996 Bowden1981
Gish1979
Menton1988
Taylor1992
Baker1976
Taylor1995
Lubenow2004
Taylor1996
Line2005
ER 1813 510 cc Ape Ape Ape Ape Ape Ape Human
Java 940 cc Ape Ape Human Ape Ape Ape Human
Peking 915-1225 cc Ape Ape Human Ape Human Human Human
ER 1470 750 cc Ape Ape Ape Human Human Human Human
ER 3733 850 cc Ape Human Human Human Human Human Human
WT 15000 880 cc Ape Human Human Human Human Human Human

In the above table, the column headers denote different creationist publications — see [Cuozzo1988; Gish1985; Mehlert1996; Bowden1981; Gish1979; Menton1988; Taylor1992; Baker1976; Taylor1995; Lubenow2004; Taylor1996; Line2005]. In the second column, the “cc” figures denote the size of the brain cavity in cubic centimeters.

The utter disagreement in this table as to whether fossils are “ape” or “human” is moot testimony to the fact that there is no clear delineation — all are related in a family tree. As biologist Kenneth Miller observes, “Ironically, validation of our common ancestry with other primates comes directly from those [creationists] who are most critical of the idea.” [Miller2008, pg. 95].

Some recent findings

What’s more, recent studies continue to identify even more interesting hominin fossils (“hominin” species means prehuman species that are either in the line of descent that led to humans or at least are closely related to humans). Here are three examples. More are listed at Prehuman fossils.

  1. Complete Neanderthal genome from one toe bone. In December 2013, a team of researchers led by Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany announced that they had extracted the entire genome of a 130,000-year-old Neanderthal from a single toe bone found in a cave in Siberia. Sarah Tishkoff of the University of Pennsylvania commented on this development by saying, “Twenty years ago, I would have thought this would never be possible.” This data, together with other recent studies, has established that humans, Denisovans (see next item) and Neanderthals represent three different branches on a common tree that diverged roughly 600,000 years ago, but that there has been significant genetic sharing through interbreeding since then [Zimmer2013c].
  2. Denisovan fossil. In March 2010, Paabo and his team announced the discovery of a new branch of hominin, identified as the result of analyzing mitochondrial DNA (mtDNA) from a finger bone found in the Denisova region of Siberia. This species co-existed with humans until as recently as 50,000 years ago, yet is roughly twice as distant (measured in terms of the time since a common ancestor) from modern humans as Neanderthals [Wong2010a]. A follow-up study published in December 2010, based on an entire genome sequence of the specimen, found that not only do the Denisovans represent a “sister” species to Neanderthals, but that in fact this race of prehumans evidently interbred with Southeast Asian humans, since the genomes of modern-day New Guinea natives contain 4.8% Denisovan DNA [Zimmer2010a]. What’s more, an additional study published in August 2011 noted that interbreeding with Neanderthals and Denisovans actually boosted human immunity to viruses [McGrath2011].
  3. Oldest prehuman DNA. In a surprising new development, announced in December 2013, researchers at the Max Planck Institute retrieved DNA from an ancient hominin fossil 400,000 years old found in Spain. It is easily the oldest specimen ever to have its DNA analyzed. These researchers had expected the specimen to be a forerunner of Neanderthals, but its DNA more closely resembles that of the Denisovan lineage, mentioned above. This raises the possibility that the Spanish specimen might belong to yet another branch of ancient prehumans, or even the remnant of Homo Erectus, which originated roughly 1.8 million years ago but was thought to be extinct more recently. Either way, researchers are puzzled and excited by the new discovery [Callaway2013b; Zimmer2013b]. For additional discussion of DNA evidence for evolution, see DNA.

Summary

In summary, there is no truth to the claim by creationists and others that there are huge gaps (i.e., “missing links”) between ancient apes and humans. If anything, there is an embarrassment of riches. So many specimens have been found in the past decade or two that the only challenge is clearly establishing their positions in the family tree, i.e., deciding which are distinct species and which are simply varieties of a single species, and which are truly in the direct line that leads to modern humans and which are evolutionary dead ends. Thus claims of “missing links” or “gaps” are only valid in the sense that when one transitional fossil is found, this creates two more “gaps” — one on each side of the new fossil!

What’s more, as can be readily seen from the examples mentioned above, those writers who still advance the “missing link” or “gap” line to criticize evolution cannot take refuge in ignorance. Most of the recent hominin fossil findings have been widely publicized, not only in scientific journals, but also in news sources such as The New York Times, Scientific American, National Geographic and New Scientist, which are available at almost any newsstand and also online. Even a quick Internet search yields numerous easily accessible and very readable articles on these discoveries.

For additional discussion, see Prehuman fossils.

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