Walking on two legs is an evolutionary leap that led humans to conquer the world. But, why humans are walking on two legs? It’s still unclear. Now, according to a new study published in the University of Chicago’s Journal of Geology, the reason might be exploding stars a few million years ago.

It was long argued that a rapid decrease in forestation in northeast Africa may have been a factor in the evolution of hominin bipedalism, but what caused that deforestation was not clear.

In a nutshell, the new study, published by Adrian L. Melott (Department of Physics and Astronomy, University of Kansas), and Brian C. Thomas (Department of Physics and Astronomy, Washburn University) says:

  • About 7 million years ago, a series of moderately nearby stars began going supernovae and blasting powerful cosmic rays in all directions. The Earth also showered with this radiation.
  • Around 2.6 years ago, it peaked.
  • These cosmic rays caused ionization of the atmosphere and made it more conductive, leading to more frequent lightning.
  • More frequent lightning caused an increase in nitrate deposition and the frequency of wildfires.
  • The wildfires would have contributed to the transition from forest to savanna in northeast Africa, making adaptations (i.e. bipedalism) vital.

Walking on two legs – a vital adaptation

Human ancestor walking on two legs
As the forests shrank, it would have become harder and more time-consuming for individuals to find food. Being able to walk on two legs became vital.

We already know that the hominin bipedalism (bipedalism: walking on two legs) evolved around the same time as these events.

As the forests shrank, it would have become harder and more time-consuming for individuals to find food.

At this point, there are a number of theories about how our ancestors started walking on two legs.

One theory, suggested by anthropologist C. Owen Lovejoy of Kent State University, says “a mutually beneficial arrangement evolved: males gathered food for females and their young and in return females mated exclusively with their providers. To be successful providers, males needed their arms and hands-free to carry food, and thus bipedalism evolved.”

Another theory considers the efficiency of upright walking. In the 1980s, Peter Rodman and Henry McHenry, both at the University of California, Davis, suggested that “…as forests shrank, hominid ancestors found themselves descending from the trees to walk across stretches of grassland that separated forest patches”. The most energetically efficient way to walk on the ground was on two legs, Rodman and McHenry argued.

There are also numerous other explanations about what caused human bipedalism.

When we first walked on two legs?

Ardipithecus ramidus
The earliest hominid with the most extensive evidence for bipedalism is the 4.4-million-year-old Ardipithecus ramidus. In 2009, researchers announced the results of more than 15 years of analysis of the species and introduced the world to a nearly complete skeleton called Ardi. Image: Scientific paleoartist Jay Matternes’ rendition of Ardi.

The earliest hominid with the most extensive evidence for bipedalism is the 4.4-million-year-old Ardipithecus ramidus. This species is best known from a partial female skeleton that got a lot of buzz in 2009. She was nicknamed “Ardi”.

Ardi was small – just under 1.2 meters tall. Based on the fossils of animals found around her, she lived in a wooded environment.

Ardi had an opposable big toe like modern apes including chimpanzees have, but the rest of the preserved bones of her foot suggest that her foot didn’t work like a grasping hand, as it does in living apes. Instead, her foot seems to have acted as a rigid lever, propelling her forward the way that the feet of humans do.

Along with her strange feet, Ardi’s skeleton also included a pelvis and this part of her anatomy had another unique combination of features.

The top of Ardi’s pelvis looked a bit like that of Australopithecus afarensis (Latin: “Southern ape from Afar”, an extinct hominin that lived between 3.9 and 2.9 million years ago in Africa): short and broad, with the beginnings of a bowl shape.

But, the bottom of her pelvis resembled the pelvis of a climbing ape, like a chimpanzee, with an angled muscle attachment point for powerful hamstrings.

So, her discoverers think that she was both a climber and a biped: an adept climber in the trees, but also an effectively walked on two legs on the ground.

Fossilized footprints have proved that human ancestors were already striding across the landscape 3.6 million years ago. But who started them on that path? What species pioneered this style of locomotion? Who was the first to walk?

The Geological Evidence of Supernovae

Previously, Melott and Thomas also contributed to a 2016 study titled “Terrestrial effects of nearby supernovae in the early Pleistocene”, published in The Astrophysical Journal Letters.

According to this study, the geological evidence confirms that multiple supernovae happened at distances of ~100 pc, consisting of two main events: one at 1.7-3.2 million years ago, and the other 6.5-8.7 million years ago.

These events are said to be responsible for excavating the Local Bubble in the interstellar medium and depositing iron-60 (60Fe) on Earth and the Moon (supernova explosions emit a wide range of metals).

Seabed sediment layers of the isotope iron-60 (60Fe) support the hypothesis.

A supernova is a violent explosion of a star that occurs under two principal scenarios.

The first (Type I) is that a white dwarf star, which is the remnant of a low-mass star that has exhausted its nuclear fuel, undergoes a thermonuclear explosion after its mass is increased beyond its Chandrasekhar limit by accreting nuclear-fuel mass from a more diffuse companion star (usually a red giant) with which it is in a binary orbit.

The Chandrasekhar limit is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about 1.4 solar masses.

The second (Type II), and about threefold more common scenario occurs when a massive star (about 12-35 times more massive than the sun), usually a supergiant at the critical time, reaches nickel-56 in its core nuclear fusion (or burning) processes. Without exothermic energy from fusion, the core of the pre-supernova massive star loses the heat needed for pressure support and collapses owing to the strong gravitational pull. The energy transfer from the core collapse causes the supernova display.

Type II supernova
Artist’s impression of a Type II supernova explosion involves the destruction of a massive supergiant star. A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion.

Abstract of the study

Abstract of the study, “From Cosmic Explosions to Terrestrial Fires?”

Multiple lines of evidence point to one or more moderately nearby supernovae, with the strongest signal at ∼2.6 Ma. We build on previous work to argue for the likelihood of cosmic ray ionization of the atmosphere and electron cascades leading to more frequent lightning and therefore an increase in nitrate deposition and wildfires. The potential exists for a large increase in the prehuman nitrate flux onto the surface, which has previously been argued to lead to CO2 drawdown and cooling of the climate. Evidence for increased wildfires exists in an increase in soot and carbon deposits over the relevant period. The wildfires would have contributed to the transition from forest to savanna in northeast Africa, long argued to have been a factor in the evolution of hominin bipedalism.


M. Özgür Nevres

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